emacs: Use `cl-lib' instead of deprecated `cl'

emacs: Use `cl-lib' instead of deprecated `cl'

Starting with Emacs 27 the old `cl' implementation is finally
considered obsolete.  Previously its use was strongly discouraged
at run-time but one was still allowed to use it at compile-time.

For the most part the transition is very simple and boils down to
adding the "cl-" prefix to some symbols.  A few replacements do not
follow that simple pattern; e.g. `first' is replaced with `car',
even though the alias `cl-first' exists, because the latter is not
idiomatic emacs-lisp.

In a few cases we start using `pcase-let' or `pcase-lambda' instead
of renaming e.g. `first' to `car'.  That way we can remind the reader
of the meaning of the various parts of the data that is being
deconstructed.

An obsolete `lexical-let' and a `lexical-let*' are replaced with their
regular variants `let' and `let*' even though we do not at the same
time enable `lexical-binding' for that file.  That is the right thing
to do because it does not actually make a difference in those cases
whether lexical bindings are used or not, and because this should be
enabled in a separate commit.

We need to explicitly depend on the `cl-lib' package because Emacs
24.1 and 24.2 lack that library.  When using these releases we end
up using the backport from GNU Elpa.

We need to explicitly require the `pcase' library because
`pcase-dolist' was not autoloaded until Emacs 25.1.