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

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

Change-Id: I810162a9dff9ea6f5d6cd7e2ebdfe486951f84c7