Add support for App Clips

[brentleyjones]

Embeds App Clips into the containing app when they are a dependency.

[brentleyjones]

@yonaskolb This is an Xcode 12 only feature. How do we want to go about testing this, since the fixture will fail when tested with Xcode 11?

[dflems]

@brentleyjones: Maybe project.yml could be templated (with mustache or some other template engine) to selectively ignore certain sections of the file unless a particular version of Xcode is selected?

[dflems]

I fiddled with it a little locally. This might require a new project yml file :\

[brentleyjones]

@yonaskolb FYI, we aren't actually testing this with CI because of the Spectre issue, but all of the infrastructure is in place for when we do add it.

xcode: ["11"] # TODO: add 12_beta (blocked by https://github.com/kylef/Spectre/issues/41)

[yonaskolb]

Looks good to me. I restarted the PR CI build, I'm not sure why it failed.
Also can we add onDemandInstallCapableApplication to the project spec docs

[brentleyjones]

@yonaskolb This is green now.

[dflems]

🙌

Thanks @yonaskolb!

w.r.t the Spectre issue: in Xcode 12, XCTest now tracks with the latest version of macOS SDK but the SwiftPM default deployment target is 10.10 so test libraries that use XCTest get a little... weird. Gonna take another stab at trying to properly fix today.

[dflems]

@yonaskolb @brentleyjones

It looks like both Spectre and XcodeGen build fine on Xcode 12 beta 4 but GitHub Actions doesn't support it yet. They're currently targeting build 12A8169g, which is beta 3 (link). Not sure when they're planning on supporting it, but once that's upgraded, we should be able to turn on the CI that will exercise these new codepaths.