Add methods returning version dependent list of types to the Dom

Currently the Dom creates a list of the names of all exports in a major version.
If then one asks for a type not available in that version the empty DomItem is returned.

Code completion wants all available completions, and it would be slow to check every entry, so the correct list should be returned in the first place.

Concretely ModuleScope.exports.keys() should return the correct list.