Remove external_go_types flag and make this the default

[bufdev]

We want to encourage this method of adoption going forward, and it is strictly cleaner in my view. Generating grpc/twirp alongside go was always a hack IMO.

[amckinney]

This isn't that crazy - this means the plugin will always generate code that will import the Go types from another package. If a user ever has a compelling reason to generate the code in the same package, we could roll out a flag that does the opposite (e.g. use_go_package or something along those lines), but we might never need to.

[akshayjshah]

Making this the default definitely makes sense, I can open a PR for that. Do we really want to remove this option altogether?