The new config loader requires some steps to happen in a different
order, particularly in regard to knowing the schema in order to
decode the configuration.
Here we lean directly on the configschema package, rather than
on helper/schema.Backend as before, because it's generally
sufficient for our needs here and this prepares us for the
helper/schema package later moving out into its own repository
to seed a "plugin SDK".
We need to share a single config loader across all callers because that
allows us to maintain the source code cache we'll use for snippets in
error messages.
Nothing calls this yet. Callers will be gradually updated away from Module
and Config in subsequent commits.
Martin Atkins