Portage/Dependency/Normalize.hs: added repeated stabilization combiner

Merged 'combine_use_counterguards' into 'combine_use_guards' pass.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Portage/Dependency/Normalize.hs: be more eager at flattening contexts

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>