Makefile.package.in: make <pkg>_SOURCE optional

For packages where the sources are included in buildroot (E.G. makedevs).
We unfortunately already use no <pkg>_SOURCE to mean <pkg>_<version>.tar.gz,
in several packages (and for git/svn/bzr support), so you need to
define <pkg>_SOURCE to the empty string to use it.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>