Store if individual source code files have been generated

For files inside source code packages it would be interesting to store if they have been generated from another source code file or not and, if possible, a reference to the origin of that file. An example would be source code files that are generated from a protobuf specification. An example Python file that I have here has the following header:

```
# Generated by the protocol buffer compiler. DO NOT EDIT!
# source: update_metadata.proto
```

In scancode there already is some code that processes some of these files and tags them:

https://github.com/nexB/scancode-toolkit/blob/develop/src/summarycode/generated.py

but it does not store a reference to the original source or how it was generated (in this case, with `protoc`). I think this would be useful information to have.