Idea - Leverage pip.dep for the coverage tooling

This is just a thought and it might not work but maybe it would.

1. Define a `pip.dep` with an extra alias to have an executable `filegroup` which is what the toolchain requires. This probably requires `pip.dep` to accept extra args.
2. If users attempt to run `bazel coverage` they would get an error asking them to include the `coverage` in the requirements.txt.

Start by examining how pip.dep handles extra arguments and how the coverage toolchain requires an executable filegroup. Compare that with the requirements.txt flow for bazel coverage; done means establishing whether the proposed alias and missing-dependency error are feasible and defining the required behavior.

索引モデルが issue の本文から書いたものです。