Add new Lint/DuplicateRequire cop

[fatkodima]

This cop checks for duplicate requires and require_relatives.

I ran it on 30k files and it detected 13 offenses, so looks useful.

This cop does not detect cases like require 'foo' vs require './foo' and the like. Looks like those will be very rare cases and not sure if it is worth the complexity.

[bbatsov]

One thing to keep in mind is that some requires might be conditional (e.g. in some if, instead at the top-level), but those cases are rare and we probably shouldn't worry about them too much.

[marcandre]

Good stuff, thanks! 🎉