cachedir: add new dir option that's default root to other dirs

buildstream/_context.py

buildstream/data/userconfig.yaml

buildstream/plugintestutils/runcli.py

conftest.py

doc/bst2html.py

doc/sessions/running-commands.run

tests/artifactcache/cache_size.py

tests/artifactcache/expiry.py

tests/artifactcache/junctions.py

tests/artifactcache/pull.py

tests/artifactcache/push.py

tests/frontend/pull.py

tests/integration/pullbuildtrees.py

tests/integration/shellbuildtrees.py

tests/integration/source-determinism.py