Please support excluding certain paths from vendor/ in Gazelle

[jmillikin-stripe]

This is probably easiest to solve as part of #859

https://github.com/kubernetes/client-go has unusual dependency handling. It vendors most of its dependencies, but leaves some deps un-vendored. This prevents Gazelle from generating BUILD files, because some externals need to use //vendor/foo.com/bar and others need @com_foo_bar//.

It would be nice if Gazelle (via go_repository) could be told that certain external dependencies should be resolved as Bazel deps, even if the overall repository is having BUILD files generated with vendored deps.

[jayconrod]

I think #859 will fix this if Gazelle is run in external mode. Dependencies that can be resolved using the vendor directory will be, anything unresolved will go to an external dependency.

I'll leave this open to be sure to verify this case when that issue is closed.

[jayconrod]

Closing old Gazelle issues. This should be working now. If not, please file a new issue at bazelbuild/bazel-gazelle.