Fix layering_check with path stripping and textual_hdrs

[keith]

Previously textual_hdrs paths were eagerly fetched which breaks path
stripping. This wires these headers up separately to treat them like
hdrs.

Fixes bazelbuild/bazel#27718

[keith]

i chose to split these out from additional_exported_headers because that field is undocumented and publicly unused. If googlers validate it wouldn't break them we could instead change the type of that argument to be a list of Files instead of a list of strings.

[keith]

note that the core difference is below the paths are already strings, where here we are processing file objects in a context where their paths can be stripped by bazel

[keith]

ptal @fmeum @pzembrod

[fmeum]

Thanks for the fix!

Cc @c-mita as the original author

[keith]

@c-mita @trybka @pzembrod friendly ping

[keith]

@c-mita @trybka @pzembrod friendly ping

[armandomontanez]

Need to run through a few rounds of presubmit testing to make sure the previous failures don't show up again. thanks for the ping.

[keith]

thanks!