Prevent addition of redundant protobuf imports

[phlax]

description

we have a lot of redundant import warnings when generating protos

i have raised a pr to fix this upstream in cncf/xds (cncf/xds#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 protocolbuffers/protobuf#3980)

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

[phlax]

im wondering if one of the protobuf linters would pick this up (and perhaps improve proto formatting in general)

im also seeing that the problem is in part because of the guidance here - https://github.com/envoyproxy/envoy/blob/main/api/STYLE.md#adding-an-extension-configuration-to-the-api

cc @htuch

[htuch]

It should be lintable by something that understands proto descriptors. I think we do something like this in the tooling to add/remove imports, but it would be better to rely on standard tooling if possible; updating the STYLE.md would help.