This allows building UWP plugin examples with `build-examples --winuwp`. As with previous pre-stable-template desktop support, this avoids the issue of unstable app templates by running `flutter create` on the fly before trying to build, so a template that will bitrot doesn't need to be checked in.
Also adds no-op "support" for `drive-examples --winuwp`, with warnings about it not doing anything. This is to handle the fact that the LUCI recipe is shared between Win32 and UWP, and didn't conditionalize `drive`. Rather than change that, then change it back later, this just adds the no-op support now (since changing the tooling is much easier than changing LUCI recipes currently).
This required some supporting tool changes:
- Adds the ability to check for the new platform variants in a pubspec
- Adds the ability to write test pubspecs that include variants, for testing
Part of https://github.com/flutter/flutter/issues/82817
Packages are the primary conceptual object in the tool, but currently they are represented simply as Directory (or occasionally a path string). This introduces an object for packages and:
- moves a number of existing utility methods into it
- sweeps the code for the obvious cases of using `Directory` to represent a package, especially in method signatures and migrates them
- notes a few places where we should migrate later, to avoid ballooning the size of the PR
There are no doubt other cases not caught in the sweep, but this gives us a foundation both for new code, and to migrate incrementally toward as we find existing code that was missed.
common.dart is a large-and-growing file containing all shared code,
which makes it hard to navigate. To make maintenance easier, this splits
the file (and its test file) into separate files for each major
component or category.
