project.el: A project has only one main root now

project.el: A project has only one main root now

Practice shows that the vast majority of projects only use one main
root.  The users of this API very often make this assumption as well.
The rest of the "roots" should be possible to express through
project-external-roots.

* lisp/progmodes/project.el: Update the commentary.
Only 4 non-obsolete generics now.
(project-root): Replacement for `project-roots'.
All callers updated.  Implementations too.
(project-roots): Declare obsolete.
(project-external-roots): Simplify the docstring.
(project-ignores): Update the docstring.
(project-find-regexp): Omit the second arg to project-files.
(project--dir-ignores): Simplify.
(project-compile): Simplify, remove outdated comment.

* lisp/cedet/ede.el: Add a FIXME.