Closes https://github.com/flutter/flutter/issues/167667.
In this PR:
- `FlutterFeaturesConfig` reads from the global config, current
pubspec.yaml, and environment
- `FeatureFlags` now delegates "was this configured" to
`FlutterFeaturesConfig`
- `featureFlags` (`Context`) now is created with knowledge of the
current `pubpec.yaml`, if any
Once this lands and we play with it a bit (i.e. migrate
`disable-swift-package-manager`), we can publicize it and update website
documentation (it won't make it into this beta/stable release anyway).
I did a significant rewrite of `feature_test`, as lots of what was being
tested no longer made sense.
