Always strip /external/<repo_name> from proto import path

[hlopko]

Before this PR, when strip_import_prefix was not specified, and the proto target
was declared in an external repository, Bazel generated import path that leaked
external/<repository_name> path fragment. That path fragment is an
implementation detail of Bazel's execroot layout and should not be used in proto
source files.

This PR removes this path fragment from the import path.

Fixes #8030.

[Yannic]

@hlopko Thanks for working on this!

I've tried this, and it seems to fix the issue with proto_library.
However, it looks like cc_proto_library, and possibly ProtoInfo#proto_source_root, also need to be fixed. java_proto_library already works with this change.

Below is an example for cc_proto_library. I'll have a look whether there's a bug in my starlark rule that uses ProtoInfo#proto_source_root or if that needs to be fixed Bazel-side.

proto_library(
    name = "some_proto",
    srcs = ["some.proto"],
    import_prefix = "add_prefix",
)

cc_proto_library(
    name = "some_cc_proto",
    deps = [":some_proto"],
)

$ bazel build @private_repo//redacted:some_cc_proto
... 
in BazelCcProtoAspect aspect on proto_library rule @private_repo//redacted:some_proto: header 'bazel-out/darwin-fastbuild/bin/external/private_repo/redacted/_virtual_imports/some_proto/add_prefix/redacted/some.pb.h' is not under the specified strip prefix 'external/private_repo/external/private_repo/redacted/_virtual_imports/some_proto'

[hlopko]

I'll look into cc_proto_library.

I don't plan to fix proto_source_root since it's being removed anyway (#7153).

[RNabel]

@hlopko Just checked these changes against the repro case in #8030 and can confirm that it fixes the error. Thanks a lot for looking into this.

[Yannic]

I was talking of this proto_source_root which, IIUC, isn't part of the deprecation in #7153.
Anyway, I've had time to investigate the issue, and it seems to be caused by a difference in the generated descriptor files when the deprecated proto_source_root attribute is used. I'm going to remove the offending targets from my code :).

[hlopko]

Updated the PR with the fix for cc_proto_library.

[oquenchil]

LGTM

[Yannic]

Thanks for looking into this! cc_proto_library also works now.