Fix #25 and one more regression.

* Don't build the source package twice (the first time around empty).
* Only try to remove packaging folder when it exists.
Minor cleanup.

* Rename Package.prepopulatePackagingDir() to populatePackagingDir(), as it is no longer
  done before everything else.
Fix regression introduced by changed source package creation.

* Filter out source packages when activating build packages for the
  install & test phase, too.
Pass '-f' to xz to avoid warning about existing files.