Support metadata-only changes

[banderous]

Scenario:

• New package version published with a metadata-only change, eg. a DLL's import settings are changed to be windows only.
• User has modified local file
• GetPack strips the incoming file from the updated unitypackage since GetPack only considers Asset changes, not metadata.

GetPack should patch the incoming unitypackage, replacing the incoming file with the local file so that the incoming metadata is applied.