use-package: Add ':ignored-files' support to ':vc' keyword

use-package: Add ':ignored-files' support to ':vc' keyword

* lisp/use-package/use-package-core.el (use-package-split-when):
New utility function to split a list whenever a specified
predicate returns t.
(use-package-vc-valid-keywords): A new defconst to gather all
allowed keywords.
(use-package-normalize--vc-arg): Properly normalize the
':ignored-files' keyword, in that the following are all valid
ways of entering files:
  :ignored-files "a"
  :ignored-files ("a")
  :ignored-files "a" "b" "c"
  :ignored-files ("a" "b" "c")
(use-package-normalize/:vc): Adjust normalization, now that we
do not necessarily receive a valid plist as an input.  (Bug#66567)

* test/lisp/use-package/use-package-tests.el
(use-package-test-normalize/:vc): Add tests for ':ignored-files'
keyword.