Prevent addition of redundant protobuf imports

description

we have a lot of redundant import warnings when generating protos

i have raised a pr to fix this upstream in cncf/xds (https://github.com/cncf/xds/pull/3)

i have previously removed them from the envoy codebase (#15329 ) and have just opened another PR to remove more recently added ones (#15588 )

it would be great if we prevented these from being added

the obvious way would be to turn warnings into errors (see https://github.com/protocolbuffers/protobuf/issues/3980)

the problem with this is that there are a lot of warnings about missing dir paths which i think is harder to fix (https://github.com/bazelbuild/bazel/issues/7157)