* nix-env -i: instead of breaking package ties by version, break them

  by priority and version install.  That is, if there are multiple
  packages with the same name, then pick the package with the highest
  priority, and only use the version if there are multiple packages
  with the same priority.

  This makes it possible to mark specific versions/variant in Nixpkgs
  more or less desirable than others.  A typical example would be a
  beta version of some package (e.g., "gcc-4.2.0rc1") which should not
  be installed even though it is the highest version, except when it
  is explicitly selected (e.g., "nix-env -i gcc-4.2.0rc1").

* Idem for nix-env -u, only the semantics are a bit trickier since we
  also need to take into account the priority of the currently
  installed package (we never upgrade to a lower priority, unless
  --always is given).