Added support for site-local packages
This makes things easier for 0compile: instead of putting builds in the cache
and registering them with the interface, 0compile can now just drop a build in
~/.local/share/0install.net/site-packages/URI/VERSION and 0install will find it
and add it automatically. The build can be removed by simply deleting the
directory.
In order to support the case where 0compile compiles a binary on a platform
with an old system 0install, writer.save_interface() adds all such discovered
feeds, tagged as 'site-package'. 0install >= 1.9 ignores such entries when
reading.