Makes extension imported by part files be accessible inside the part file, not to the entire library. (Existing phrasing didn't need to  distinguish the two.)

Makes closer imports in the parent-file chain be more specific than further away imports.

This ensures that a part file that imports all its own dependencies, including implicitly referenced extensions, can ignore parent imports and the extensions those parent imports make accessible, and not risk a conflict or override by an equally or more specific extension.

Fix typos