Add bp2build converter for python protobuf files

Bug: 196084681
Test: b run //build/bazel/examples/python/protobuf:build_bazel_examples_python_protobuf_main --config=linux_x86_64
Change-Id: I4d806902d262351231f64686a5d24513a25d9749

bazel/properties.go

@@ -652,6 +652,11 @@ func MakeLabelListAttribute(value LabelList) LabelListAttribute {
 	}
 }
 
+// MakeSingleLabelListAttribute initializes a LabelListAttribute as a non-arch specific list with 1 element, the given Label.
+func MakeSingleLabelListAttribute(value Label) LabelListAttribute {
+	return MakeLabelListAttribute(MakeLabelList([]Label{value}))
+}
+
 func (lla *LabelListAttribute) SetValue(list LabelList) {
 	lla.SetSelectValue(NoConfigAxis, "", list)
 }