[bazel] `cc_grpc_library` incompatible with `proto_library` that has `strip_import_prefix`

I believe this likely has the same root cause as https://github.com/grpc/grpc/issues/28199.

When I attempt to use `cc_grpc_library` with a `proto_library` that has a `strip_import_prefix`, there is some sort of conflict where protoc attempts to compile both the original proto path and the proto path from within virtual imports (which I assume is an effect of having a `strip_import_prefix`).

I tried making a few combinations of tweaks to the protoc rules but when I managed to make it not pick up the non-virtual-import proto file, the output was then generated to the incorrect location as well, so I am unsure what the fix would be. (Most of this proto virtual import stuff is very mysterious to me)