Edebug: Overload `edebug-form-spec` even less

Edebug: Overload `edebug-form-spec` even less

The `edebug-form-spec` symbol property was used both to map forms's
head symbol to the corresponding spec, and to map spec element names
to their expansion.

This lead to name conflicts which break instrumentation of examples such as

    (cl-flet ((gate (x) x)) (gate 4))

because of the Edebug spec element `gate`.
So introduce a new symbol property `edebug-elem-spec`.

* lisp/subr.el (def-edebug-elem-spec): New function.

* lisp/emacs-lisp/edebug.el (edebug--get-elem-spec): New function.
(edebug-match-symbol): Use it.
(Core Edebug elems): Put them on `edebug-elem-spec` instead of
`edebug-form-spec`.
(ELisp special forms): Set their `edebug-form-spec` via dolist.
(Other non-core Edebug elems): Use `def-edebug-elem-spec`.
(edebug-\`): Use `declare`.

* lisp/emacs-lisp/pcase.el (pcase-PAT, pcase-FUN, pcase-QPAT):
* lisp/skeleton.el (skeleton-edebug-spec):
* lisp/emacs-lisp/cl-macs.el: Use `def-edebug-elem-spec`.

* test/lisp/emacs-lisp/edebug-tests.el
(edebug-tests--conflicting-internal-names): New test.
* test/lisp/emacs-lisp/edebug-resources/edebug-test-code.el
(edebug-test-code-cl-flet1): New test case.

* doc/lispref/edebug.texi (Specification List): Add `def-edebug-elem-spec`.
(Specification Examples): Use it.

* doc/lispref/loading.texi (Hooks for Loading): Avoid the use of
`def-edebug-spec` in example (better use `debug` declaration).