We were unconditionally inserting into the yaml cache anything we had
retrieved from the yaml cache.  This had the side-effect of being expensive
on re-runs of commands where yaml hadn't changed.

Signed-off-by: Daniel Silverstone <daniel.silverstone@codethink.co.uk>

_yaml.py: Do not insert into cache if retrieved from cache

See merge request !902

WorkspaceLocal contains all the information needed to generate data
that's stored within the workspace. Currently this is just a list of
project directories.