Consider adding top-level `source` property

[dglazkov]

Two benefits:

1) Allows us to have a simple way to validate source and library (check to see if discovery #13 URL and source URLs match)

2) Can make chunk URLs relative.

[jkomoros]

One challenge is for libraries that include chunks from multiple sources. For example a load_default_libraries merges together multiple libraries in out/

This is the kind of place having Library be a class helps, we could easily abstract over the underlying data structure