Build shallow renderer independently

[gaearon]

Alternative strategy for #9372 which shouldn't suffer from injection issues. My concern here is I'm not sure what happens if components use react-dom (which isn't deduplicated anymore) but I guess any other solutions suffer from the same problem, and we don't really interact with react-dom in shallow tests anyway.

[bvaughn]

Tested this build:

• Ran npm run build to build modules (without errors).
• Can I make a new project with only react + react-test-renderer and some tests? Yes.
• Can I use both react-test-renderer and react-test-renderer/shallow in the same tests? Yes.
• Then I copied react + react-dom into react-virtualized and verified that all the unit tests passed and the demo site seemed to work correctly.

[gaearon]

Wow I fixed a bug! Not every day I get to do this with React. 😄