[OE-core] any way to list *all* possible sources across the current layers?

[Robert P. J. Day]

   hoping this question makes sense -- i use own-mirrors.bbclass to point
at a sizable directory of tarballs (almost 12G worth) that i've been
collecting over many months and, after every OE layer upgrade, whatever
newer sources are pulled in, will generate the corresponding tarball and
i'll copy that into my tarballs directory.

   predictably, at this point, there's a *lot* of old, irrelevant cruft in
that directory, but i know of no automated way to get rid of any of it.
for instance, i currently have five versions of automake (1.11.2, 1.12.1,
1.12.3, 1.12.5 and 1.12.6) taking up space.  doesn't hurt, of course, but
it would be handy to be able to generate a list of all possible,
potentially necessary versions of a source -- that is, any version for
which any recipe still lists it as PREFERRED_VERSION.

   does that make sense?  obviously, that would have to be done in the
context of a given set of layers and all the corresponding recipes in
those layers.  is there a way to do that already?  if not, i'm sure i
could whip up a simple script that would do the job.  thanks.

rday