.flowconfig option module.name_mapper can't map to a lib definition

Repro:

.flowconfig

```
[lib]
lib
[options]
module.name_mapper='1Doesnt\(Exist\)' -> '\1s'
```

A.js

``` javascript
/* @flow */
var m1 = require('./1DoesntExist'); // actual error: can't find module 1DoesntExist
var a_1: string = m1.numVal; // expected error: number ~> string
```

lib/Exists.js

``` javascript
declare module 'Exists' {
declare var exports: {
numVal: number;
};
};
```

Expected output:

```
A.js:3:19,27: number
This type is incompatible with
A.js:3:10,15: string
```

Actual output:

```
A.js:2:22,37: ./1DoesntExist
Required module not found
```

Workaround:
While libdefs aren't working, it's still possible to map to a node module. There's an npm module called `empty` that can be useful here. See [this gist](https://gist.github.com/lambdahands/d19e0da96285b749f0ef) for a complete example.

I think the issue is that we do name mapping specifically against file paths, so the code just doesn't attempt to handle this case. I think that it should. cc @jeffmo, who originally added this feature.