In Flutter docs, classes are listed multiple times

[Hixie]

When a library exports another library, but both libraries are being docced together, the result is that each class that is exported by both libraries is listed twice.

We should only list classes as coming from the package that they actually come from, and not mention them in packages that re-export them, if we are including that package in the docs.

See also:

• don't link to exported name if original location is public and inside same package #1090 (about where the links to reexported symbols should point)
• Exported classes don't have any indication where they come from #936 (about saying where types come from when reexported)
• For Flutter docs, be more aggressive about including dependencies #1159 (about automatically documenting even non-exported libraries)

cc @devoncarew

[devoncarew]

also adding @keertip

[abarth]

As an example, the BoxDecoration class has four different URLs:

https://docs.flutter.io/flutter/painting/BoxDecoration-class.html
https://docs.flutter.io/flutter/rendering/BoxDecoration-class.html
https://docs.flutter.io/flutter/widgets/BoxDecoration-class.html
https://docs.flutter.io/flutter/material/BoxDecoration-class.html

It's not obvious that these are the same class. If there was a canonical URL that all the re-exports linked to, that would make that much clearer.