Turn Eldoc, Xref and Project into GNU ELPA :core packages

Turn Eldoc, Xref and Project into GNU ELPA :core packages

The new packages state they require Emacs 26.3 to function, but a
small part of project.el breaks this "soft" rule: the two functions
requiring fileloop.el are incompatible with Emacs 26.3.

* lisp/jsonrpc.el: Tweak comment near Package-Requires.

* lisp/emacs-lisp/eldoc.el: Add Version and Package-Requires.

* lisp/progmodes/flymake.el: Add comment near Package-Requires.

* lisp/progmodes/project.el: Add Version and Package-Requires.

* lisp/progmodes/xref.el: Add Version and Package-Requires.