Refactor artifact log command

See merge request !1101

Most split rules already included the relevant directories themselves in
addition to the directory contents.  Add the missing bin, sbin, and
libexec directories.

This is required to fix tests with the following commit that changes
list_relative_paths() to return all directories.

This is required to fix tests with the following commit that changes
list_relative_paths() to return all directories.

Returning only empty directories leads to inconsistencies when computing
a manifest by combining results from multiple list_relative_paths()
calls as done by the compose plugin.

I.e., the same directory may be empty in one dependency and non-empty in
another dependency.  The merged file list will still contain that
directory even though it's no longer empty.

This inconsistency causes problems when calculating differences between
manifests. Returning all directories fixes these inconsistencies.

This is a change in API behavior.

This matches the change in utils.list_relative_paths().

Return all directories in list_relative_paths()

See merge request !1139

_context.py: Add cache_buildtrees global user context, the default
of which is set to by default to 'always' via the addition of
cache-buildtrees to userconfig.yaml cache group. 'failure' & 'never'
can be given as valid options.

app.py & cli.py: Add --cache-buildtrees as a bst main option, which
when passed with a valid option can override the default or user
defined context for cache_buildtrees.

tests/completions/completions.py: Update for the added flag.

not _cached_sucess() could resolve to true if the element wasn't
cached at all. switch to _cached_failure() to ensure condition
reflects expected artifact state

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

element.py: _cache_artifact() Check if context for cache_buildtrees
has been set to always or failure with a corresponding build
result, if not 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 and failure option.

NEWS: Add entry for new option.

Optional creation of buildtrees

Closes #896

See merge request !1135

Makes artifactdir and builddir obsolete.

Fixes #870

Will check and move old artifact directory if it exists, and create
symlink linking old directory to new.

This sits in Context allowing artifact cache to check the cas quota
while not being used for CASServer.

A lot of code that checks cache quota has been touched.

Part of #870