This introduces a new GjsModule class for imported module objects. The
functionality is unchanged, but we will use this as a basis for making
bindings from modules' lexical scopes (variables declared with 'let' and
'const') available for compatibility. In SpiderMonkey 45, these will stop
being exported as properties, as per ES6 standard.

The function GjsModule::evaluate_import() contains duplicated code with
gjs_eval_with_scope(), but that will change when we have to import the
lexical scope separately.

https://bugzilla.gnome.org/show_bug.cgi?id=781429