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.

This fixes a case where, in project overrides, if a user specified
a scalar/sequence instead of a mapping for a given project, the
user would receive an ugly stacktrace instead of an invalid format.

By avoiding this, loading metaelements of junctions becomes cheap even
for junctions with erroneous dependencies, and we can ignore their
task reporting.

Task reporting for junction metaelement loading is confusing, since
the junction element itself will never be part of the pipeline, so
we'd rather not have this show up as an actual loaded element.

Elements loaded from the junction are loaded separately, therefore
this does not affect their progress display.

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

Test that we get the correct provenance from a node which has
been compositied.

This tests asserts that if we forget to include one of the
local plugin's essential keys, we fail and see the appropriate
error message.

Tests issue described in  #1059.

Continuing moving plugins over to bst-plugins-experimental.