Consolidate code duplicated in ArtifactCache and SourceCache

The following discussion from !1635 (merged) should be addressed:

• @juergbi started a discussion: (+1 comment)

  This makes sense. However, it seems we have some code duplication between ArtifactCache and SourceCache with differences in error handling. We should fix this with a method in CASCache that fetches a directory with file blobs (or adding a parameter to the already existing _fetch_directory()) and possibly a helper method in the BaseCache class. The former can be implemented with a single FetchTree call to buildbox-casd in the future.

  Do you want to update this branch or defer this to a separate MR?