Crash if element filename contains '@'

BuildStream constructs the element name from its filename, and in turn constructs an OSTree ref name from the element name.

The Baserock definitions contain some chunks with '@' in their name. It seems '@' is invalid in OSTree ref names, leading to a cras when checking the cache:

  File "/usr/lib/python3.5/site-packages/buildstream/element.py", line 719, in _cached
    self.__cached = self.__artifacts.contains(self)
  File "/usr/lib/python3.5/site-packages/buildstream/_artifactcache/artifactcache.py", line 100, in contains
    return _ostree.exists(self.repo, ref)
  File "/usr/lib/python3.5/site-packages/buildstream/_ostree.py", line 203, in exists
    ref = checksum(repo, ref)
  File "/usr/lib/python3.5/site-packages/buildstream/_ostree.py", line 255, in checksum
    _, checksum = repo.resolve_rev(ref, True)
GLib.Error: g-io-error-quark: Invalid refspec baserock/genivi-genivi-demo-platform-audiomanager@gdp/2d8556592e6eee107a1d36ee287f1c31260d6b8b476af427c96e416c90ae998c (0)

Full log at: https://gitlab.com/baserock/definitions/-/jobs/29346832