Invalid CI packaging setup, race condition / missing multitarget matrix

[Arlodotexe]

Background

MultiTargets & WinUI support

The toolkit supports the following targets, each supporting a specific WinUI version:

MultiTarget	WinUI
uwp	2
wasdk	3
wasm	2, 3
android	2, 3
ios	2, 3
macos	2, 3
linuxgtk	2, 3
wpf	2, 3
netstandard	n/a

Intended Behaviors

Packaging should "just work" regardless of combination of supported targets, such as:

• uwp
• wasdk
• netstandard
• uwp,netstandard
• wasdk,netstandard
• uwp,wasdk
• uwp,netstandard,wasdk

The CI currently builds WinUI 2 and 3 simultaneously, creating package variants for both:

• CommunityToolkit.Uwp.* (uwp multitarget)
• CommunityToolkit.WinUI.* (wasdk multitarget)

Problems

Some components support only one of uwp or wasdk, plus a non-winui target like netstandard, such as uwp,netstandard or wasdk,netstandard. This creates a race condition:

1. Building wasdk first can produce a package with netstandard but without uwp.
2. Building uwp first can produce a package with netstandard but without wasdk.

Further, packages with neither uwp nor wasdk (e.g. targeting netstandard only) won't be built via CI at all, since our "packaging step" only targets WinUI 2 and 3 with its matrix.

Solutions

One of the listed problems could be patched by deciding to build either UWP or WinUI first instead of doing it simultaneously, but there's a few reasons we don't. To understand why, consider:

• A component supporting uwp,netstandard could be fixed by not allowing wasdk to build first.
• A component supporting wasdk,netstandard could be fixed by not allowing uwp to build first.

Both cannot be done, meaning we must fix the issues with concurrent builds directly.

We'll need two things:

• An added CI matrix for packaging netstandard-only components properly.
• When building WinUI 2 or 3, skip unsupported components.
  • Skip building any component that targets only netstandard.
  • Skip building uwp,netstandard components if packaging for wasdk.
  • Don't skip building wasdk,netstandard components if packaging for wasdk.
  • Skip building wasdk,netstandard components if packaging for uwp.
  • Don't skip building uwp,netstandard components if packaging for uwp.

These can be split into two tickets, only the latter is high priority right now as it unblocks AppServices in Labs.