The artifact cache emits messages, and we want to allow that in preflight.

We can streamline this call to os.statvfs() in a few places.

This will benefit from a better UtilError being raised, and
and turns the artifact cache's local function into a one liner.

The loop which finds the first existing directory in the
given path has been removed, being meaningless due to the
call to os.makedirs() in ArtifactCache.__init__().

The local function was renamed to _get_cache_volume_size() and
no longer takes any arguments, which is more suitable for the
function as it serves as a testing override surface for
unittest.mock().

The following test cases which use the function to override
the ArtifactCache behavior have been updated to use the new
overridable function name:

   tests/artifactcache/cache_size.py
   tests/artifactcache/expiry.py

A simple object which creates a snapshot of current
usage statistics for easy reporting in the frontend.

A frontend facing API for obtaining usage statistics.

I would have put this on Stream instead, but the Context
seems to be the de facto place for looking up the artifact cache
in general so let's put it here.

This also adds some comments around the main status bar heading
rendering function.

Added some useful status messages when:

  * Calculating a new artifact cache usage size

  * Starting a cleanup

  * Finishing a cleanup

Also enhanced messaging about what was cleaned up so far when
aborting a cleanup.

Updates the known cache size in the main process while the cleanup
process is ongoing, so that the status indicators update live
while the cleanup happens.

This seems to have been copy/pasted from cascache, and
documents the function to possibly return None if defer_prune
was specified, but this function does not expose defer_prune.

Cache management logging enhancements

See merge request !1105