Make WRKOBJDIR for py*- pkgs version specific?

[Martin Husemann <martin%duskware.de@localhost>]

Can we please make the WRKOBJDIR for multi-version pkgs include the version
name?

That is: if I build a few pkgs and on the way there run into py27-expat
and py37-expat as dependencies (due to me setting PYTHON_DEFAULT_VERSION=37
and some pkg not accepting that), it would be great if not both of them
would try to use ..../pkgobj/textprox/py-expat as the work dir.

Instead they could use ..../pkgobj/textprox/py27-expat and
..../pkgobj/textprox/py37-expat.

Fallout from re-use of the directory is obvious, the .*_done marker files
prevent a rebuild and the whole build breaks when trying to install the
second dependency from the same objdir.

Martin