Eglot: load built-in GNU ELPA dependencies explicitly

Eglot: load built-in GNU ELPA dependencies explicitly

Because of outstanding bug#62576, it's way to easy for users to get
confused by the M-x package-install process for Eglot.

Whenever an Eglot release depends on a new version of a GNU ELPA :core
package which is _also_ provided built-in, M-x package-install will
install that dependency, but will not always load it on top of the
built-in one.

The solution is to not use "require" for now.  This may potentially
lead to double "loads", but that should in principle be idempotent.

* lisp/progmodes/eglot.el (project, eldoc, seq, flymake, xref, jsonrpc)
(external-completion): Load, don't require.