Portage/Dependency/Types.hs: changed 'DependIfUse' to accept simpler type as USE switch

Introduced simpler type
    data DUse = DUse (Bool, Use)
      driving (Eq, Show)

To disallow incorrect representations like
    foo ( a b )
 or
    =bar ( a b)

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