go.mod update support

[bigkevmcd]

It'd be nice (especially if migrating a project between orgs) to be able to run gomove and have it change the go.mod if necessary.

Not sure if this should be an optional thing, or something you explicitly don't want.

Basically, I'm thinking that running gomove github.com/myorg/my-repo github.com/neworg/my-repo would patch all the Go files, and also, update the module too (unless otherwise requested).

module github.com/myorg/my-repo => module github.com/neworg/my-repo