Currently drive-examples determines what plaforms a package supports by looking at plugin metadata, which means it only works for plugins. This change skips that check for platforms, and adds per-example filtering based on what platforms the example itself supports.

This should allow enabling drive-examples in flutter/packages.

Also does some minor refactoring of existing utilities to leverage the newer RepositoryPackage class, which will make repeated checks in this logic more efficient since RepositoryPackage caches parsed pubspec data.

Part of flutter/flutter#102851

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.