Centralize all checks for .packages and pubspec.yaml

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.