Allow setting search paths for imports.
Set import path to directory of main module.
Set the module search path for imported modules.
Document import in ast_support.md.
Store path of loaded module in visitor.
Ensure a module is loaded from the same path as parent.
Separate __modules__ from __registry__.

Previously, if a __registry__ entry was a function, it was
executed to turn it into a module. Now, there's a separate
object that holds the imported modules.
Modules no longer need to return __module__.
Allow importing __main__ from package.