Handle `--absdeps` correctly

Fixes #122.

For -A and -M commands, --absdeps now tries to sync and build targets
and dependencies that are not already installed. These ABS queries are
done using pacman instead of precomputing the ABS tree. Split packages
are still not handled, but are passed to pacman instead of failing.

Problems this does not fix:
* Split packages (issue #125)
* `-(A|M)d --absdeps` without `sudo` can be misleading, since all ABS syncs fail and the dependencies are passed to pacman instead of being built
* `-A[d] --absdeps` lists all AUR and ABS packages as AUR packages. `label` needs to be moved out of `InstallOptions` and into `Buildable`