Reworked the PackMap datastructure to store the selected pack by key,
not object.
That way if the object is replaced in the map the new object at that key
is retrieved instead of the old reference.

Signed-off-by: Dalton Pelc <daltonjpelc@gmail.com>
Fixed another place where the launcher was not checking if the directory
was empty before selecting it as an install location.

Signed-off-by: Dalton Pelc <daltonjpelc@gmail.com>
Merge branch 'master' of https://github.com/TechnicPack/TechnicLauncher