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 https://github.com/bazelbuild/bazel/issues/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]

ptal @fmeum @pzembrod

[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!