In-place pack update could leave pack in an invalid state

[mgziminsky]

When updating an existing pack installation by providing a new URL, the new pack is unzipped directly over the existing installation without first cleaning up the contents of the previous pack. This can result in multiple versions of mods, or other files, from the overrides folder being installed.

To avoid this, a simple solution would be to iterate the contents of the previous pack.zip and remove them from the installation before unpacking the new one. This would account for any changes to the new pack while preserving most other modifications made by the user or MC.

A more robust solution, possibly dealing user changes to the manifest or config probably falls under #39