The private _ libraries should not be advertised as importable. Moving them to the src directory.

I noticed this while browsing our dart docs. This should remove the underscore entries.

![image](https://github.com/flutter/flutter/assets/1227763/41511efe-4eb8-487a-9758-ccde1720e05a)

Replaces https://github.com/flutter/flutter/pull/137197.