module.name_mapper to map absolute import paths no longer works

This .flowconfig:

```
[options]
module.system=haste
module.name_mapper='^\/\(.*\)$' -> '/src/\1'
```

used to make imports like this work:

`import Header from '/components/Header'`

Now the above setup results in errors:

```
src/components/App.jsx:5
5: import Header from '/components/Header'
^^^^^^^^^^^^^^^^^^^^ /components/Header. Required module not found

Found 1 error
```

Changing the prefix away from `/` to anything else results in the same error.

Also, in order to debug this more, is there any way to ask Flow to be more verbose about exactly where it's looking for modules?