rustdoc_test: Apply prefix stripping to proc_macro dependencies.

[konkers]

Without stripping the prefix, rustdoc can not find the proc macro shared library.

It's not entirely clear to me why //test/unit/rustdoc:lib_with_proc_macro passes without this. When I run bazel test //test/unit/rustdoc:rustdoc_test_suite --subcommands I don't see any "action 'Generating Rustdoc test runner for.." lines so perhaps that path is not tested?

[UebelAndre]

Thanks! Seems reasonable. I can look at the existing test but what would fail before this change and can that be added as a regression test?

[konkers]

Thanks for the look. I created an stripped down example repo to illustrate the failure: https://github.com/konkers/bazel-rustdoctest-proc-macro-failure

[konkers]

I noticed in #1782 an additional change is also added:

        root = proc_macro_dep.crate_info.output.root.path
        if not root in roots:
            roots.append(root)

Perhaps that should be added to this PR.

[armandomontanez]

I spun up a test that covers this at konkers#1

[konkers]

I spun up a test that covers this at konkers#1

Merged into PR

[armandomontanez]

@UebelAndre this is ready for another look

[krasimirgg]

@UebelAndre this is looking reasonable now, with a regression test in place.
@konkers just to clarify:

I noticed in #1782 an additional change is also added

This PR is good as-is, and doesn't require this?

[UebelAndre]

Thank you!