Extend xref-file-name-display to elisp and etags definitions

Extend xref-file-name-display to elisp and etags definitions

And all other types of locations (with a looks-like-file-name check).

* lisp/progmodes/xref.el (xref--group-name-for-display): Extract
from xref-buffer-location's implementation of xref-location-group.
(xref-file-location): Define trivial reader for the 'file' slot.
(xref-location-group): Update docstring.
(xref--analyze): Use the new function here, to be able to format
group names coming from any location type.