proto: support looking up by package

[jmillikin-stripe]

The proto.MessageType and proto.GetProperties functions can, together, be used to introspect the fields available to a protobuf message type identified by name. This request is for corresponding functionality that operates at the level of protobuf packages.

syntax = "proto3";
package k8s.io.some.package;

message SomeMessage { ... }
message OtherMessage { ... }

The API doesn't need to be very complex, just enough to figure out which messages are available:

type Package struct {
  Name string // "k8s.io.some.package"
}
func (*Package) MessageTypes() []reflect.Type

// []string{"k8s.io.some.package.SomeMessage", "k8s.io.some.package.OtherMessage"}
func (*Package) MessageNames() []string

Use case: I want to add type safety to an existing protobuf-compatible config system. The user-visible behavior and error messages will be much more pleasant if the code can introspect the set of messages available to a named Protobuf package.

[dsnet]

Hi, thanks for the issue. I'm going to close this as a duplicate of #268, which is the super-bug, tracking registration improvements. What you are describing is a feature I would like to see be possible in the overhaul of registration.