Unhidden filenames with forbidden characters in subdirectories now
correctly fail the build instead of silently being skipped.
Previously this behavior would only trigger on files in the root of
the embedded directory.

Fixes #54003
Change-Id: I52967d90d6b7929e4ae474b78d3f88732bdd3ac4
Reviewed-on: https://go-review.googlesource.com/c/go/+/670615


Reviewed-by: Michael Matloob <matloob@google.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>