…ists for each package

This is a workaround for facebook/flow#4917.

**Note**: self-provided defs will only be found for packages that are already installed in `node_modules`!
It also enables package authors to update their type defs without having to make PRs to `flow-typed`.
If an installed package provides its own `flow-typed` directory it will override anything in `flow-typed`'s central repo.

### Directory structure
A package's `flow-typed` directory has the same structure as `flow-typed/definitions/npm/<package>_v<version>/` would,
except that the `.js` files must use the exact version of the package:
```
node_modules/underscore/
  flow-typed/
    flow_v0.13.x-v0.37.x/
      underscore_v1.8.3.js
    flow_v0.38.x-/
      underscore_v1.8.3.js
```

…ests