As discussed over the mailing list, reformat code using Black. This is a
one-off change to reformat all our codebase. Moving forward, we
shouldn't expect such blanket reformats. Rather, we expect each change
to already comply with the Black formatting style.

It seems we don't need this anymore, thanks to cleaning up gRPC
background threads.

This is now split into storage/index remotes, where the former is
expected to be a CASRemote and the latter a BuildStream-specific
remote with the extensions required to store BuildStream artifact
protos.

Additionally, test code that was previous executed by a subfunction (in
the forked process) has been folded into the test function itself, as
separating it is now redundant. This removes some duplicate code for
setting up the context and project, etc.

It's essential to call preflight() when loading/resolving the Elements.
This patch moves the preflight call to new_from_meta, so that it is
called as soon as the Element is created. This avoids the need for
having multiple callsites.

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