Provide configuration for the optional creation of buildtrees

Artifacts can cached explicitly with an empty `build tree` when
built via the cli main options or user config. Default behaviour
is to still create and cache expected buildtrees.

element.py: _cache_artifact() Check if context for
dont_cache_buildtrees has been set to true, if so skip attempting
to export the build-root. Element types without a build-root are
cached with an empty buildtree regardless. Update
_stage_sources_at() to warn the user that the buildtree import is
empty.

tests/integration: Add test to artifact.py for the optional caching
of buildtree content from bst build. Rename build-tree.py to
shellbuildtrees.py to reflect included test cases, add test for
empty buildtree warning.

NEWS: Add entry for new option.