Add appengine/gcloud/mustache dependencies.

pkg/appengine/lib/src/protobuf_api/internal/api_base.proto

+// Built-in base types for API calls. Primarily useful as return types.
+
+syntax = "proto2";
+
+package appengine.base;
+
+message StringProto {
+  required string value = 1;
+}
+
+message Integer32Proto {
+  required int32 value = 1;
+}
+
+message Integer64Proto {
+  required int64 value = 1;
+}
+
+message BoolProto {
+  required bool value = 1;
+}
+
+message DoubleProto {
+  required double value = 1;
+}
+
+message BytesProto {
+  required bytes value = 1 [ctype=CORD];
+}
+
+message VoidProto {
+}