[gogenproto] Allow specifying a Go package mapping for import paths

[bhollis]

Background

→ It may be the case that you're importing protos that are generated in some other way than with gogenproto but which similarly have a conventional Go path mapping (e.g. they don't specify go_package but instead get automatic package names. Unfortunately right now that means you can't depend on these protos from your gogenproto-using protos, since you can't tell gogenproto what their package name will be.

Changes

• Add an optional Go package prefix mapping to import paths. By appending = and the Go package prefix, you can use conventional (directory based) path mapping.

Testing

• Tested in stately monorepo