Normalize.hs: fix further dependency simplification

The code tried to simplify original condition,
not the next iteration.
Dependency/Normalize.hs: fix 'remove_empty' pass, implement basic 'flatten' pass

'remove_empty' pass wirked incorrectly for cases like
    DependAnyOf [ SATISFIED, something ]

before patch it resulted in
    DependAnyOf [ something ] -> something

after the patch it gives
    SATISFIED

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