Allow `bzl_library` to depend on non-`bzl_library` targets
Notably, `filegroup`. `bzl_library` doesn't actually read anything from the `StarlarkLibraryInfo` provider, and requiring all deps to be other `bzl_library` targets is really painful for anyone loading .bzls from `@bazel_tools` or `@platforms` because those core modules/repos don't want a dependency on Skylib just for access to `bzl_library`.
The medium-term plan will be to move `bzl_library` into `@bazel_tools`; but before then, this can serve as a stop-gap.
Co-authored-by: Alexandre Rostovtsev <arostovtsev@google.com>
Alexandre Rostovtsev