Revisit default behavior for go_package_prefix in managed mode

[bufdev]

When I have a dependency on a buf module, I'd expect the default behavior to use remote generation, but it doesn't right now (which was a decision we made). We should either document this better, or change this in a v2 config.

[amckinney]

This might be something we want to re-explore when the plugin redesign is complete (cc @smallsamantha), either in the form of documentation or a version: v2. I imagine we'd lean towards documentation changes for now.