xref: return absolute paths when possible

[vemv]

Although clojure-emacs/cider#3349 is a good idea (I'm building on top of it), its technique of translating relative, unprefixed paths like "orchard/cljs/meta.cljc" sadly is not workable, because it only sometimes works.

cider-find-file's docstring says:

Return a buffer visiting the file URL if it exists, or nil otherwise.
If URL has a scheme prefix, it must represent a fully-qualified file path
or an entry within a zip/jar archive. If AVFS (archive virtual file
system; see online docs) is mounted the archive entry is opened inside the
AVFS directory, otherwise the entry is archived into a temporary read-only
buffer. If URL doesn't contain a scheme prefix and is an absolute path, it
is treated as such. Finally, if URL is relative, it is expanded within each
of the open Clojure buffers till an existing file ending with URL has been
found.

Because it only translates relative paths for open buffers, it will often return nil.

Therefore, whenever possible, xref middleware ops have to return file:<absolute-path>.

cider-find-file's translation still seems desirable for some cases, mainly TRAMP and Docker.