Cas refactor
Description
After discussion with @juergbi, it's been realised that not all of !1013 (closed) is necessary for the source cache implementation, and it's worth. This does part of #802 (closed) that's more important for source cache, but leaves the strict split of CASRemote and CASCache.
Changes proposed in this merge request:
- Move
_artifactcache.artifactcacheto `_artifactcache - Rename
_artifactcache,_casas it will no longer be used solely by artifact cache. - Move methods in
CASCachethat are purely remote.
This merge request is related to #802 (closed) and #440 (closed)