Warning when registering plugin using baseDir if broccoli-rollup is installed

When attempting to add a plugin
```
setupPreprocessorRegistry(type, registry) {
registry.add('htmlbars-ast-plugin', {
baseDir() {
return __dirname;
},
});
},
```

if `broccoli-rollup` is installed then when running `ember serve` I get the message:
`Warning: failed to stat /preprocess-rollup/node_modules/broccoli-rollup/tmp/rollup-input_base_path-6obEeBLr.tmp/0`

If I remove `broccoli-rollup` or remove the `baseDir`
```
setupPreprocessorRegistry(type, registry) {
registry.add('htmlbars-ast-plugin', {});
},
```

Then no warning is printed.

This is reproducible in a brand new addon like https://github.com/jrjohnson/preprocess-rollup (addon doesn't do anything and the plugin itself won't work since it doesn't exist, bu the message is obvious.

I'm running into this while working on https://github.com/FortAwesome/ember-fontawesome/pull/21