Centralize all checks for .packages and pubspec.yaml

[jacob314]

Bazel projects do not use pubspec.yaml or .packages files so all checks for pubspec.yaml and .packages need to be carefully audited to make sure we aren't breaking bazel support. We should define a PackageInfo class with a custom implementation for Bazel projects that wraps all cases where we need to perform logic that checks for pubspec.yaml, .packages with solutions that are appropriate under Bazel as well.

[alexander-doroshko]

Please keep me updated about the progress. We'll probably need to add the same changes to the Dart plugin. We need to be sure that the Flutter and Dart plugins agree which one is responsible for the pub actions bar in the pubspec file, which 'Outdated Dependencies' inspection should work, etc.