Detect proto.Message types when failing to export a field

[dsnet]

While we do not provide 1st class handling of protobuf messages, we can at least detect whether the type is a proto.Message and refer the user to the correct package to use to handle protobuf messages with cmp.Equal.

[dsnet]

Unfortunately, I can't add a test for this as doing so would require a dependency on google.golang.org/protobuf.

However, here's the code in action:

package main

import (
	"github.com/google/go-cmp/cmp"
	"google.golang.org/protobuf/types/descriptorpb"
)

func main() {
	m1 := &descriptorpb.DescriptorProto{}
	cmp.Equal(m1, m1)
}

panic: cannot handle unexported field at {*descriptorpb.DescriptorProto}.state:
		"google.golang.org/protobuf/types/descriptorpb".DescriptorProto
	consider using "google.golang.org/protobuf/testing/protocmp".Transform to compare proto.Message types

[dsnet]

Note that MethodByName could massively increase a binary size, but cmp is only used for testing, so this isn't a major detriment. Also, golang/go#62257 makes it such that string literals passed to MethodByName are specially handled such that only those particular methods must be linked in, while the entire universe of all methods can be mostly DCE'd.