Fix scope resolution for Google namespace

[Parad0X]

Second stab at #4167

Having google in the package namespace generates unusable ruby code.

syntax = "proto3";

package foo.vendor.google;

message Bar {
  int32 baz = 1;
}

# Generated by the protocol buffer compiler.  DO NOT EDIT!
# source: test.proto

require 'google/protobuf'

Google::Protobuf::DescriptorPool.generated_pool.build do
  add_message "foo.vendor.google.Bar" do
    optional :baz, :int32, 1
  end
end

module Foo
  module Vendor
    module Google
      Bar = Google::Protobuf::DescriptorPool.generated_pool.lookup("foo.vendor.google.Bar").msgclass
    end
  end
end

which obviously doesn't work:

<module:Google>: uninitialized constant Foo::Vendor::Google::Protobuf (NameError)

Google::Protobuf::DescriptorPool must be prefixed with :: to fix this.

[Parad0X]

@xfxyjwf @haberman ping since you guys approved #4167 :)

[Parad0X]

@haberman tests fixed

[Parad0X]

@haberman ping