BUTCH_SKIPLIST works only partially

[rofl0r]

the intention is to completely ignore items in the skiplist.
this works with regard to adding them not to the buildqueue, however it does not work if they are dependencies of another package.
so for example BUTCH_SKIPLIST=gcc3:stage0_gcc:stage0_musl:patch butch install gdb (gdb depends on patch, which depends on some stage0 stuff) will silently fail to build if the patch entry is not found in butch.db, as its listed as dependency of gdb.
possible fix is to iterate over all pkgs in both queues before starting them and removing any from deps that are in the skiplist.