`bst-artifact-server` no longer works

Summary

Since the landing of !1071 (merged), we're no longer able to initiate an artifact server. See below:

 $ bst-artifact-server
Traceback (most recent call last):
  File "/home/jennis/.local/bin/bst-artifact-server", line 5, in <module>
    from buildstream._artifactcache.casserver import server_main
ModuleNotFoundError: No module named 'buildstream._artifactcache.casserver'; 'buildstream._artifactcache' is not a package

Obviously, this module no longer exists.

To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information