The addition of the pull-buildtrees flag will be used to override
the default behaviour of remote artifact cache pulling. Included
is the required changes to completions tests.

The addition of the pullbuildtrees option to the users global
buildstream.conf will be used to override the default behaviour
of remote artifact cache pulling.

The addition of cached buildtrees being included in element
artifacts has led to mostly redundant download overheads when
pulling from a remote artifact server. As such the default behaviour
of pull shouldn't fetch the buildtree object if available.

_stream.py: propagate if pullbuildtrees has been set via cli or
user config to the PullQueue.

pullqueue.py: add extensible attributes to determine required/
excluded directories for element pull jobs.

element.py: extend relevant pull logic for specified subdir
consideration and ensure push logic does not lead to partial
artifact pushes.

_artifactcache/: artifactcache.py & cascache.py inclusion of
helper functions for subdir artifact checking & fetching, fetch
logic extended to only pull required artifact directories.
extract & checkout updated to handle full/partial operation

tests/: integration test pullbuildtrees.py & fix for testutils/
artifactshare.py pathing.