Check collisions between Alternatives and files/links
We should check for collisions between alternatives and other files. Right now we just overwrite any existing link in place but that is not ideal.
The collision checking should discover following problem: turris/turris-build#122
We should also handle the same way all links. We can insert links from packages with priority 0. That way we can fallback on them if Alternatives package is removed and no other alternative (over just original package) was left in system.