Link command for Cosmos contributors

[ovidiuch]

A watch command that Cosmos contributors can use to link the Cosmos monorepo to another test repository on the same machine. Basically watching for changes in packages/$pkg/dist/(**/*) and automatically copying the changed files to /path/to/test/repo/node_modules/$pkg/dist/$1.

yarn/npm link doesn't work because some Cosmos modules look up dependencies at run-time relative to where the Cosmos modules are placed.

Command would look like this:

yarn link-dist /path/to/test/repo