[Tooling] Improve Emission scripts to support testing of dependency updates

[javamonn]

In #109 we introduced an initial set of scripts to support testing palette changes in emission locally.

In implementing #135 I realized that it's difficult to test changes involving palette dependencies locally in emission. The added scripts drop the compiled output into emission/node_modules/@artsy/palette (as Metro does not support symlinks), but does not update package artifacts: emission/node_modules/@artsy/palette/node_modules remains the versions of dependencies that the published version of palette is using. This is a little counterintuitive, as I'm used to updating a dependency in a linked package and rebuilding the consumer to test the changes out.

I wasn't able to run yarn install --production manually within emission/node_modules/@artsy/palette/node_modules either after copying over a changed package.json either due to Metro module map errors. I'm not sure but I wonder if it's due to the hard React dependency being incompatible - is there a reason we don't have a peerDependency on React?