Merge.hs: fix breakage on undesolved depends

Reverts bogus 5fca958de2c9fbb9f77866e37fe0c2330e60d81d commit.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Portage/Dependency/Normalize.hs: added stub pass for 'combine_use_counterguards'

It's able to merge
    a? ( foo )
    !a? ( foo )
into
    foo

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>
Portage/Dependency/Types.hs: slightly changed USE sort order

Now 'test' has most priority among other USEs.
Makes ebuilds look a bit nicer.

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