Spotted by Darius.

Now that both are equivalent, we can skip the sanitization part before
the yaml call.

This changes the location of source refs from `cache/cas/refs` to
`cache/sources_protos` and fixes tests that looked in the old reference
place. Source cache now uses the new protocol buffer and grpc methods
defined, for storing locally, and pushing and pulling to remotes.

Part of #1038

Instead of having methods in Context forward calls on to the Messenger,
have folks call the Messenger directly. Remove the forwarding methods in
Context.

Remove the need to pass the Context object to message handlers, by
passing what is usually requested from the context instead.

This paves the way to sharing less information with some child jobs -
they won't need the whole context object, just the messenger.

Calling '_yaml.dump' will itself call '_yaml.node_sanitize', therefore
we can remove all calls to it in places where we directly after call
dump.

This will replace the previous use of a directory structure.
Quite a lot is changed here, predominantly _artifact and _artifactcache
modules.

Part of #974

- Rename plugintestutils to testing.

- Don't run the tests from bst-plugins-template. This imports
  buildstream.plugintestutils so will have to be disabled to get
  through CI. This can be re nabled once bst-plugins-template has been
  patched.

This needs to be exposed as part of the plugin author facing API so
that plugin authors can define custom repo types which will can be
passed to the set of tests which iterate over multiple source types.

Part of the work towards #944