Ensure .dir-locals-2.el behavior as documented (bug#75890)

Ensure .dir-locals-2.el behavior as documented (bug#75890)

* lisp/files.el
(dir-locals--all-files): New &optional 'base-el-only' argument.
(dir-locals--base-file): New function.
(dir-locals-find-file): 'locate-dominating-file' only for the base
.dir-locals.el.

* test/lisp/files-tests.el
(files-test-dir-locals-2-solo): New test.
* test/lisp/files-resources/dir-locals-2-solo: New test support.
(files-test-dir-locals-2-paired): New test.
* test/lisp/files-resources/dir-locals-and-2: New test support.

(cherry picked from commit 81c21d89ede8dfa664b7a3700acd7bf4c9fa54aa)