Merge "Allow generated source to be root module src." am: 69cd8ed5c5 am: b5c54b485c am: 03b33f40b5 am: 372389cee1

Original change: https://android-review.googlesource.com/c/platform/external/rust/crates/protobuf-codegen/+/1412814

Change-Id: I1e0fd5fe622106e638787d9eeed4d6ab3141a2a6