ProjectRepairer: Refactor to extract the repository lookup

copyPackTo() validates the destination URI and opens the repository to
locate it on disk before copying anything, neither of which has to do
with packs.

Move both into objectsDir(), which reports the repository's objects
directory, or nothing if the project cannot be repaired at that
destination. What is left is copyPacksTo(), taking the directory to
copy from. The now trivial copyInOrder() folds into it.

Change-Id: I2d1ba0a23671d6db115f18a35420f4c04a24c0ec