Summary:
`react-native start` already ensures that the `--transformer` path is understood to be relative to CWD, not to the module that ends up importing that file. `react-native bundle` and `react-native dependencies` didn't up until this point.
**Test plan:** Ensured that `react-native bundle ... --transformer ./relative/path` works with this patch applied.
Closes https://github.com/facebook/react-native/pull/7857
Differential Revision: D3393777
fbshipit-source-id: 303a226fae9c8087c3dd3b2e8d004462ca66665e