all: trivial formatting changes

Changes:
* import grouping for third-party dependencies
* import grouping for generated protobufs
* blank space removal

Change-Id: I2950b0606bb2064046d79a23a78b05c23147cbfe
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/221017
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/internal/impl/message_reflect_test.go b/internal/impl/message_reflect_test.go
index 26c271d..66c2536 100644
--- a/internal/impl/message_reflect_test.go
+++ b/internal/impl/message_reflect_test.go
@@ -15,6 +15,7 @@
 
 	cmp "github.com/google/go-cmp/cmp"
 	cmpopts "github.com/google/go-cmp/cmp/cmpopts"
+
 	"google.golang.org/protobuf/encoding/prototext"
 	pimpl "google.golang.org/protobuf/internal/impl"
 	"google.golang.org/protobuf/proto"