Some renaming.

Use xref-lookup-FOOS for backend functions and
xref-find-FOOS for commands.

* xref.el (xref-lookup-definitions): Renamed from
xref-find-definitions.
(xref-find-definitions): Renamed from xref-find-definition.
(xref--find-definitions): Renamed from xref--find-definition.

Add missing (provide 'xref).
Add option for alternative UI.

The variable xref-show-xrefs-function can be used
to display multiple xrefs in a different way.

* xref.el (xref-show-xrefs-function): New variable.
(xref--show-xref): Factorized from xref--find-definitions.
(xref--show-xref-buffer): Call xref--analyze here.
(xref--unique-location): Split out from xref--analyze.
(xref--analyze): Only find common filenames.
(xref--find-definitions): Now a thin wrapper around
xref--show-xrefs.
(xref-find-references): New command.