Improve/clean up genproto rules

[Code0x58]

While upgrading to python3, the bazel protobuf rules took some coercing to get around protocolbuffers/protobuf#1491. A better solution should be used to clean up code and improve stability†. It seems to me that these are options, by order of preference:

• using standard protobuf bazel rules and their import_prefix
• pursue Add replace_import_package Python option. protocolbuffers/protobuf#7470
• use relative paths in the protoc command so the imports are absolute (mentioned in issue)

† the build may flake due to files being removed while they are being iterated over