[#4716] ResourceTypeAPI exposes implementation details that are likely to change

Made the following modifications to ResourceTypeAPI:

(1) returned nil from “define”, “hostclass”, and “node”.

(2) renamed “mk_resource_type” and “munge_type_arguments” to
“mk_resource_type” and “munge_type_arguments” to discourage
customers from calling them.

(3) Made ResourceTypeAPI a class rather than a module, and changed the
parser to evaluate the contents of pure ruby manifests using a
instances of this class.

(4) Changed ResourceTypeAPI to insert newly instantiated types into
Thread.current[:known_resource_types] rather than the default
environment's known_resource_types.

This effectively backports the fix for issue #4657 to 2.6.x.

Also backported the new spec tests from #4657.