Plugin format versioning was decided to be removed for local
plugins and any plugins for which we do not load an explicitly
provided plugin. For pip, this will be handled with a standard
distutils/setuptools approach, allowing users to specify pip
style version boundaries in the plugin origins.

This patch refactors plugin loading so that all related code
goes into the private _pluginfactory module, a new small
PluginOrigin type was added to better manipulate loaded
origins.

Test cases have been removed and will be readded in a following
commit, adjusted to new expectations.

Plugin tests are already accessing this API, but using imports from
private modules. For motivation for this to be exposed publicly, note
that ErrorDomain is an argument for most things in runcli.py, and
LoadErrorReason may be another.

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.

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.

Replace some popular copypasta.

This important-looking invocation:

    os.path.join(datafiles.dirname, datafiles.basename)

is equivalent to this shorter invocation:

    project = str(datafiles)

It seems like it's very popular copypasta, replace it with the shorter
one thus:

    # Use 'gsed' or 'sed' etc. as appropriate for your system.
    git config --global alias.sub '!f() { git grep --name-only --null "$1" | gxargs --null gsed --in-place --expression "s/$1/$2/g" ; }; f'
    git sub 'os.path.join(datafiles.dirname, datafiles.basename)' 'str(datafiles)'

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

Part of #870