Fix depset/list confusion for ctx.attr.deps.

Signed-off-by: Harvey Tuch <htuch@google.com>

protobuf.bzl

@@ -85,8 +85,8 @@ def _proto_gen_impl(ctx):
         import_flags = depset(direct=["-I."])
 
     for dep in ctx.attr.deps:
-        import_flags = depset(transitive=[import_flags, dep.proto.import_flags])
-        deps = depset(transitive=[deps, dep.proto.deps])
+        import_flags = depset(transitive=[import_flags, depset(direct=dep.proto.import_flags)])
+        deps = depset(transitive=[deps, depset(direct=dep.proto.deps)])
 
     if not ctx.attr.gen_cc and not ctx.attr.gen_py and not ctx.executable.plugin:
         return struct(