Add unused-imports tool

This adds a tool for using index data to determine what imports in your files can be removed. It currently is limited to only imports in your project (ignoring system imports) and does not add missing imports, it only removes unused ones. This means you may have to add missing imports if an import was being polluted by another otherwise unused modules. There are also a few upstream bugs in Swift that affect the accuracy of this tool, you can see the details on #5. If you try this out on your project please report any issues you find!