BSolverRepository: Add SetPriority()
pkgman: Simplify and rename resolve-build-dependencies

* Now it only gets a package (info) file and a single list of
  repository directories, optionally with priority, and resolves the
  package's dependencies. The more complex two resolving steps it did
  before can just as well be done by haikuporter, and this way the
  command is more flexible.
* Rename to resolve-dependencies.
* Some TODOs still remain.