Any objections to the attached patch?

Having a "filename" parameter for `project-find-file' is useful with
comments like:

 // see file "fancy_hack.c" for more details

Put point on "fancy_hack.c", M-x project-find-file (or your favorite key
binding), and the file comes up.


Making `project--find-file-in' generic allows my ede and ada-mode
backends to override the default implementation.


The change in xref just provides a missing doc string for a function
that is used by the default project--find-file-in.

-- 
-- Stephe