Add support for removing unused imports

[alexanderkjeldaas]

I made a similar module to this, but I don't have it anymore (and I don't own the code, my ex-employer does).

Anyways, in my module I had this simple function that just collects the import symbols and scans the source for these tokens. Imports that don't have a token in the file can be safely removed.

Saves a lot of time and keeps the import list clean.

[dakrone]

I think this is a great idea. I also think it wouldn't be too hard to do, just requires the plumbing.

Thanks for suggesting it :)