test/ebuild: only test parent repo category inheritance for SlavedTrees
pkgcore/ebuild: remove old overlay repo support

We've been using multirepo support via pkgcore.repository.multiplex
around the time portage switched to repo lookup ordering as well (around
two years ago).
use tuples for package_keywords_splitter values

This allows incremental_expansion_license to work as expected since it
requires hashable objects due to its set usage.
fix /etc/portage/package.license support
cleanup old license related code and add summary docstrings