ExtractDependencies: Split dependency recording in separate class

This will be used later in this PR to perform dependency recording during
Typer.

The new API is slightly higher level:
- Dependencies are recorded using `addUsedName`, `addUsedRawName` and `addClassDependency`.
- `addUsedName` takes a Symbol instead of a Name and takes care of name
  mangling, `addUsedRawName` is still available when there is no
  symbol (currently this only happens when recording a dependency on a renamed import).
- These methods do not take a `from` argument, instead they take care of
  calling `resolveDependencySource` internally.
- Recorded dependencies are sent to Zinc using `sendToZinc`.

Author: smarter