Use goimports fork for modules support

I've pushed a beta version of goimports that uses go/packages for module support. As usual, it's slower than we'd like it to be but should be functional. I think it's ready for use in the same circumstances as the godef and gocode forks.

It does have one slight difference in behavior vs. standard goimports, described in golang/go#28428, but that change should be merged soon.