They switched away from extracting the SDK files into the Asset folder, so you have to manually extract these into your project's Packages folder. You need Base and Avatars.
Remember to remove existing SDKs from Assets or Packages. If you want to keep compatibility with the VCC, remove the dash and version numbers (to com.vrchat.avatars).
If things don't work, delete the Library folder and let it regenerate the scripts and cache.
A unitypackage is just a compressed tarball (a different kind of "zip") anyways. It doesn't do anything special.
