Rename project-library-roots to project-external-roots

Rename project-library-roots to project-external-roots

* lisp/progmodes/project.el (project-library-roots): Rename to
project-external-roots.
(project-library-roots-function): Rename to
project-vc-external-roots-function.  Only use it in the VC
backend, for now.  Update project-external-roots accordingly.
(project-vc-library-roots): Remove.
(project-or-libraries-find-regexp):
Rename to project-or-external-find-regexp.

* lisp/progmodes/elisp-mode.el (elisp-library-roots):
Rename to elisp-load-path-roots.

* lisp/progmodes/etags.el (etags-library-roots): Remove.  Use
an anonymous function for the default value of
project-vc-external-roots-function.