[ITP] ploticus and ploticus-doc

[Andrew Schulman]

> I don't have time for a review, but shouldn't ploticus-doc have an
> external-source directive in its setup.hint?

No, I don't think so, because the docs come in a separate tarball from
the ploticus source.

However, now that you mention it, it probably would make sense to
split out libploticus from ploticus.  These are built from the same
source, so an external-source directive would apply there.  I'll do
this and repost the ITP.