Provide better error message when a plugin requires a min iOS version higher than the app

[stuartmorgan-g]

Currently the experience of trying to use a plugin that has a higher minimum iOS version than the app's minimum iOS version is really verbose, and not very easy for someone without CocoaPods experience to understand. For instance, here's the output of trying to build the google_maps_flutter example after changing the plugin to require iOS 12, but not updating the example:

[...]
Running pod install...                                             493ms
CocoaPods' output:
↳
      Preparing

    Analyzing dependencies

    Inspecting targets to integrate
      Using `ARCHS` setting to build architectures of target `Pods-Runner`: (``)
      Using `ARCHS` setting to build architectures of target `Pods-RunnerTests`: (``)
      Using `ARCHS` setting to build architectures of target `Pods-RunnerUITests`: (``)

    Finding Podfile changes
      A google_maps_flutter_ios
      R google_maps_flutter
      - Flutter
      - OCMock
      - integration_test

    Fetching external sources
    -> Fetching podspec for `Flutter` from `Flutter`
    -> Fetching podspec for `google_maps_flutter_ios` from `.symlinks/plugins/google_maps_flutter_ios/ios`
    -> Fetching podspec for `integration_test` from `.symlinks/plugins/integration_test/ios`

    Resolving dependencies of `Podfile`
      CDN: trunk Relative path: CocoaPods-version.yml exists! Returning local because checking is only performed in
      repo update
      CDN: trunk Relative path: all_pods_versions_c_0_6.txt exists! Returning local because checking is only
      performed in repo update
      CDN: trunk Relative path: Specs/c/0/6/OCMock/3.9.1/OCMock.podspec.json exists! Returning local because
      checking is only performed in repo update
    [!] CocoaPods could not find compatible versions for pod "google_maps_flutter_ios":
      In Podfile:
        google_maps_flutter_ios (from `.symlinks/plugins/google_maps_flutter_ios/ios`)

    Specs satisfying the `google_maps_flutter_ios (from `.symlinks/plugins/google_maps_flutter_ios/ios`)`
    dependency were found, but they required a higher minimum deployment target.

    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:317:in `raise_error_unless_state'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:299:in `block in unwind_for_conflict'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:297:in `tap'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:297:in `unwind_for_conflict'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:682:in `attempt_to_activate'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:254:in `process_topmost_state'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolution.rb:182:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/molinillo-0.8.0/lib/molinillo/resolver.rb:43:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/resolver.rb:94:in `resolve'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:1078:in `block in
    resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/user_interface.rb:64:in `section'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:1076:in
    `resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer/analyzer.rb:124:in `analyze'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:416:in `analyze'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:241:in `block in
    resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/user_interface.rb:64:in `section'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:240:in `resolve_dependencies'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/installer.rb:161:in `install!'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/command/install.rb:52:in `run'
    /Library/Ruby/Gems/2.6.0/gems/claide-1.1.0/lib/claide/command.rb:334:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/lib/cocoapods/command.rb:52:in `run'
    /Library/Ruby/Gems/2.6.0/gems/cocoapods-1.11.3/bin/pod:55:in `<top (required)>'
    /usr/local/bin/pod:23:in `load'
    /usr/local/bin/pod:23:in `<main>'

Error output from CocoaPods:
↳

    [!] Automatically assigning platform `iOS` with version `11.0` on target `Runner` because no platform was
    specified. Please specify a platform for this target in your Podfile. See
    `https://guides.cocoapods.org/syntax/podfile.html#platform`.

Error running pod install

It's particularly misleading because the "Error output from CocoaPods" section isn't actually the problem. Adding an explicit minimum version to the example's Podfile will make that part go away, but won't change anything else.

The actual error is right in the middle of that wall of text:

    Specs satisfying the `google_maps_flutter_ios (from `.symlinks/plugins/google_maps_flutter_ios/ios`)`
    dependency were found, but they required a higher minimum deployment target.

Not only is that line not as easy to find as we'd like, but it's not clearly actionable. It doesn't say what the plugin minimum is, what the app minimum is, or what to do next (update the app's minimum deployment target, for which that missing info would be very useful, or remove the plugin).

We should consider parsing the output and providing our own error message instead for this case.

[jmagman]

We should consider parsing the output and providing our own error message instead for this case.

Alternatively parse the podspec for all dependencies and generate the correct minimum deployment target build setting in the xcconfig, and print a message that their app minimum is x because of dependency y.

[stuartmorgan-g]

Automatically changing the deployment version seems a bit dangerous; if someone doesn't notice the message they might accidentally drop support for older versions of iOS without meaning to.

[stuartmorgan-g]

For reference, the Android equivalent to this error message is:

flutter/packages/flutter_tools/lib/src/android/gradle_errors.dart

Lines 465 to 469 in 1a51dc2

	'The plugin ${minSdkVersionMatch?.group(3)} requires a higher Android SDK version.\n'
	'$textInBold\n'
	'Following this change, your app will not be available to users running Android SDKs below ${minSdkVersionMatch?.group(2)}.\n'
	'Consider searching for a version of this plugin that supports these lower versions of the Android SDK instead.\n'
	'For more information, see: https://docs.flutter.dev/deployment/android#reviewing-the-gradle-build-configuration',

[jmagman]

cc @LouiseHsu