Use separate properties to be able to distinguish between own and
inherited ones. When "appending" to a property, it effectively copies
it do the own object, making it hard to notice whether the properties
of a parent module really are copied/preserved at each step, vs only
visible at step 1 where it copies it thus masking later bugs where
we might forget to extend/mixin the parent env.

Also add a test for the module scope callback's context object.

Ref #1762.