Please review carefully, this is my first work below the gexp-border.

This patch is the outcome of my question at:

https://lists.gnu.org/archive/html/guix-devel/2019-04/msg00226.html

When a jar-file had patches, it was not recognized and wrongly handled
with "tar xz".

There is one open question: For patch-and-repack, the flags defaults
to '("p1"), but for jar-files we would usually need a "p0". That would
mean for jar-sources with patches, one would need to manually add the
patch-flags with "p0" to the origin. This is not nice. On the other
hand, I would not like to override the patch-flags for jar-files to
always "p0".

WDYT?

Björn