[options] module_mapper not resolving on sibling projects

Hi there,

I have an issue with module resolution in a project, I reproduced the error in this repo: https://github.com/melloc01/flowconfig-module_mapper but it's simple so I'll try to explain:

The directory structure is like this:

```
> common/
.flowconfig
/core/
service.js
/modules/
widget.js

> project_1
.flowconfig
app.js
```

with the following **flowconfigs**:

**common**'s:
```
[options]
module.name_mapper='^core\/\(.*\)$' -> '/core/\1'
```

**project_1**'s:
```
[options]
module.name_mapper='^core\/\(.*\)$' -> '/../common/core/\1'
```

Both **common/modules/widget.js** and **project_1/app.js** have the same source code:

```js
// @flow
import service from "core/service";
```

but Flow trows an error on **project_1**, am I missing something?

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