Allow customization of the "install" tab

[jonasfj]

We should render INSTALL.md on the "install" tab, if the package contains an INSTALL.md file.
(motivated by #2125)

This allows packages with external dependencies to specify how they are installed. Imagine packages using dart:ffi, it also allows packages that can be globally activated to indicate that this is how they should be used. And it allows packages that prefers to be dev_dependencies to indicate this.

Moreover, it allows packages to recommend version constraint ranges, useful in cases when bump to 1.0.0 happened for cosmetic reasons and didn't cause any breaking changes (#3385).

Mostly, I think this is generally useful to allow packages to specify their own install instructions, if they want to.

@mit-mit, @kevmoo, thougths?

[kevmoo]

Not a bad idea!

I pondered if we should have a field or fields in the pubspec for this. But maybe just an MD file is better.

[jonasfj]

@sigurdm suggest we could also remove the install tab, and put succinct installation instructions on the readme tab. Preferably with an easy way to copy-to-clipboard.

[elbeicktalat]

Great it's very helpful 👌

[leighajarett]

+1 This came up in product research sessions at Flutter Vikings. A few developers mentioned that they wanted control over the install tab. Specifically they wanted to be able to mention other setup steps that are required. Right now this tends to fall in the "Getting Started" tab but developers seemed to think that should be reserved for actually beginning to write code.

[sigurdm]

I think custom information in the install tab would easily go unnoticed.

How would someone know to look there if most packages have the default instructions?

[simolus3]

How would someone know to look there if most packages have the default instructions?

Maybe there could be a UI hint for packages with providing a INSTALL.md file to put an emphasis on that tab for those packages.

However, I think problem is not that the install tab goes unnoticed - after all, the complains are coming from the install tab containing wrong information and being noticed by users.

I think custom information in the install tab would easily go unnoticed.

Most packages that require something not covered by the default instructions prominently describe that in their readme, or link to external documentation giving help on how to get started using the package. For consistency, and to not confuse users, it would good if that information could also be mentioned in the install tab. I think most package authors are well aware that the readme is the best place to put that information, and will continue to put that information there.

[anderdobo]

+1 this has come up as we assess documentation quality of Flutter Favorite candidates. We find that packages that are meant to be dev dependencies have a discrepancy between their README(e.g. https://pub.dev/packages/flutter_flavorizr) which instructs to manually add under dev_dependencies in pubspec.yaml, and the instructions on the "Install" tab (e.g. https://pub.dev/packages/flutter_flavorizr/install of the package, which provides a command that adds as a regular dependency. It would be useful to provide package authors with a mechanism that makes the instructions on the "Install" tab relevant for a dev dependency (e.g. provide the command dart pub add [package_name] --dev)

[mit-mit]

Yeah, I think this is a great idea too!

[sigurdm]

@anderdobo

We find that packages that are meant to be dev dependencies have a discrepancy between their README , and the instructions on the "Install" tab

I think we should address this by making it possible to mark a package specifically for being a dev-dependency: see #6400