use UnwatchedDir for includePaths

@jasonmit submitted an optimization to https://github.com/kellyselden/ember-cli-node-modules-to-vendor/pull/62/files to not watch for rebuild the paths that you want to include. Maybe this project could benefit from the same kind of optimization in `includePaths`.

Start by reviewing the referenced ember-cli-node-modules-to-vendor pull request and locating how includePaths are handled in this project. Confirm how UnwatchedDir is used there, then verify that included paths remain available without being watched for rebuilds.

索引モデルが issue の本文から書いたものです。