Allow libraries without sources

[valis]

It should be possible to have external libraries without sources. Even if a library has sources, we do not need to load them. We also don't have to load them for the double checker.

To implement this, it is necessary to add some additional information to serialized expressions. Referables carry some additional information. It can be partially inferred from the typechecked expressions, but I think some of the information should be stored separately.