contrib/openrc/init.d/package-cache: Don't include 'distfiles' in source

Portage will request ${MIRROR}/distfiles/${FILENAME}, so we don't want
the 'distfiles' part in the source directory.  With the shift, we can
also use a single cache for both source (distfiles) and binary
packages (PKGDIR, which defaults to /usr/portage/packages) if there
are any binary packages on the upstream mirror.
README: Document Gentoo / OpenRC usage for distfiles caching

This is what I'm using this project for ;).