feat: split doc.go generation into comments and code portions

[shollyman]

This PR splits the current content of generated doc.go into their respective code and documentation generation parts.

• doc.go - package docs, canonical import name
• helpers.go - DefaultAuthScopes helper, client construction hook definitions, getVersionClient helper

Related: b/377347282

[quartzmo]

@shollyman Bikeshed: Was there anything wrong with the simpler helpers.go name?

[shollyman]

@shollyman Bikeshed: Was there anything wrong with the simpler helpers.go name?

Collision avoidance. Looking internally there's already a number of protos named helpers.proto so I wanted to avoid having to add special collision logic.

[shollyman]

@shollyman Bikeshed: Was there anything wrong with the simpler helpers.go name?

Collision avoidance. Looking internally there's already a number of protos named helpers.proto so I wanted to avoid having to add special collision logic.

Though in retrospect, the protoc generator produces files with the .pb.go suffix so the risk of collision is smaller. I'm happy to switch back to helpers.go in that case.