Make importpath optional for go_binary and go_test

[jayconrod]

All Go rules have an importpath attribute, which is specified explicitly with the importpath attribute or is derived from go_prefix. importpath is used to determine output paths and is used by the experimental go_path rule.

We'd like to deprecate go_prefix (#721). In repositories without go_prefix, importpath is effectively mandatory, which is surprising for go_binary and go_test, since they can't be imported. When importpath isn't specified explicitly for these, we should create a dummy path. We should also report an error when rules without a real importpath are used in go_path.