fix: allow generated files to be in a subfolder

[alexeagle]

The logic for taking the basename assumes that outputs are in the same folder with the package.

This fix allows a protoc invocation like

proto_compile(
    name = "view_display_proto_compile",
    output_mappings = ["payload/view_display_pb2.py=internal/payload/view_display_pb2.py"],
    outputs = ["payload/view_display_pb2.py"],
    plugins = ["@build_stack_rules_proto//plugin/builtin:python"],
    proto = "view_display_proto",
)

Note: I only tested this with manually generated rules, I imagine changes to the gazelle plugin would also be needed.

[pcj]

Thanks @alexeagle I'll take a look at this. Outputs in subfolders would indeed be a new usage.

[alexeagle]

ping! :)

[pcj]

Sadly this is quite old. Will close due to age, but we can pick this up again if still needed.