Upgrade to golang protobuf api v2

Major operations:
 * Fix the go_package entries to be consistent, as the new tool
   complains when it doesn't have a '/'.
 * Regenerate with the new protoc-gen-go tool.
 * github.com/golang/protobuf -> google.golang.org/protobuf
 * proto.[Un]MarshalText -> prototext.[Un]Marshal

Change-Id: Ie1147bd2457fafb66ba555461b3bf14f0561a25d
diff --git a/go.mod b/go.mod
index 7297dea..9a2ebcc 100644
--- a/go.mod
+++ b/go.mod
@@ -1,10 +1,10 @@
 module android/soong
 
-require github.com/golang/protobuf v0.0.0
+require google.golang.org/protobuf v0.0.0
 
 require github.com/google/blueprint v0.0.0
 
-replace github.com/golang/protobuf v0.0.0 => ../../external/golang-protobuf
+replace google.golang.org/protobuf v0.0.0 => ../../external/golang-protobuf
 
 replace github.com/google/blueprint v0.0.0 => ../blueprint