Root cache directory

Background

Raised here (!1013 (comment 129888354)), it makes sense to move some directories out of artifact cache and into the top level cache structure, in particular cas and tmp which will be used by the source cache once that's implemented (see #440 (closed)). As many of these directories should be in the same directory we should deprecate the current builddir and artifactdir options, and have them placed in this root directory.

Task description

• Issue warnings for when artifactdir and builddir are set by a user
• Add new rootcachedir option under which tmp, cas and the default for artifactdir and builddir are placed.

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