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Returning a cache remains unchanged, but the case of returning a
file (or pattern) is now changed to return the contaning
directory.

(dir-locals-read-from-file): Rename to `dir-locals-read-from-dir'
and make obsolete.
(dir-locals-read-from-dir): Simplify accordingly.
(hack-dir-local-variables): Simplify accordingly and rename a
variable.