Rewrite bs_mirrorfull as osclib component

[Vogtinator]

This has multiple benefits:

• Easier to interface with the python code that calls it
• Uses the osc python interface for connection and authentication handling
• Allows to drop the bundled OBS code in the bs_copy directory

Draft because not tested as part of pkglistgen and installcheck yet.

[Vogtinator]

project-installcheck.py, staging-installcheck.py and pkglistgen.py work here