[proto] Add ability to inject python files when scanning protos.
[proto] Add ability to inject python files when scanning protos.

This will be used to ease the buildbucket builder.proto rename by
adding a python shim which emits warnings in place of the generated
builder.py file.

Change-Id: Id7980ee40d1d638952ac3b4c6487288a046ff56b
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/recipes-py/+/3674717
Auto-Submit: Robbie Iannucci <40f3d43a28ebae3cb819288542e1c84d73d962d5@chromium.org>
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>

luci/recipes-py,luci/recipes-py