Resolve dependencies exactly like apk

pmbootstrap's dependency resolution was not yet written with the target in mind to work exactly like apk's dependency resolution work. I went with a simple dependency resolution algorithm that appeared to be wroking, and patched it, whenever it failed. Also I rewrote it where necessary to match new assumptions.

But issues like #1862 (closed) (with @Minecrell's excellent analysis) show that it still is behaving differently from apk and, while real-world usage is good enough, it is still confusing to everybody if something does go wrong and we need to figure out why.

So let's make it work exactly like apk works, by reading the code and adapting the algorithm, and re-implementing its test cases, just like it was done for the version parsing.