Attached is a patch that implements find-file-project, with completion
of file-name on the project search path. It handles duplicate filenames
by uniquifying them witht trailing directory names.

The patch also adds small projects for elisp and global, to show that
this approach works for multiple backends.

Comments?

I can break this into smaller commits on master, if that seems like a
good idea.

I didn't add a NEWS entry. I don't think we are putting project related
changes in NEWS yet, since it is all new in Emacs 25. But the
file-name-all-completion change needs a NEWS entry.

-- 
-- Stephe