Tighten types and visibilities

[srawlins]

A variety of cleanups:

• InheritingContainer: hasPublicInheritedMethods, inheritedFields, publicInheritedFields, publicInheritedMethods are only visible for testing (should probably be tightened beyond that).
• _cacheNewModelElement's newModelElement parameter is non-nullable.
• _constructFromPropertyAccessor's two named parameters are required.
• On the allConstructedModelElements map, the library part of the key is non-nullable, as is the value.

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