Imported less files (using less-loader) not resolving.

Using flow 0.50.0 with a `.flowconfig` as follows:

```config
[ignore]
/node_modules/stylelint
/build
/notebooks

[options]
module.system.node.resolve_dirname=node_modules
module.system.node.resolve_dirname=src
module.name_mapper.extension='gql' -> 'empty/object'
module.name_mapper.extension='less' -> 'empty/object'
```

When running flow on my project, I am getting errors on any attempted `*.less` import, like:

```shell
src/universal/components/Hamburger/index.js:8
8: import './hamburger.less'
^^^^^^^^^^^^^^^^^^ ./hamburger.less. Required module not found
```