descriptor: rename generated protobuf package on import.

This works around https://github.com/google/go-genproto/issues/8,
renaming the local package to match its expected name.

descriptor/descriptor.go

diff --git a/descriptor/descriptor.go b/descriptor/descriptor.go
index df8aa76..83607ff 100644
--- a/descriptor/descriptor.go
+++ b/descriptor/descriptor.go
@@ -43,7 +43,7 @@
 	"io/ioutil"
 
 	"github.com/golang/protobuf/proto"
-	"google.golang.org/genproto/protobuf"
+	protobuf "google.golang.org/genproto/protobuf"
 )
 
 // extractFile extracts a FileDescriptorProto from a gzip'd buffer.

descriptor/descriptor_test.go

diff --git a/descriptor/descriptor_test.go b/descriptor/descriptor_test.go
index 7e4037d..282a1e3 100644
--- a/descriptor/descriptor_test.go
+++ b/descriptor/descriptor_test.go
@@ -6,7 +6,7 @@
 
 	"github.com/golang/protobuf/descriptor"
 	tpb "github.com/golang/protobuf/proto/testdata"
-	"google.golang.org/genproto/protobuf"
+	protobuf "google.golang.org/genproto/protobuf"
 )
 
 func TestMessage(t *testing.T) {