Improve imports resolution

ModuleDefinition instances do not store imported top-level names anymore.
(definitions attribute).
Instead name resolution routine recursively traverse imports of
imported module if required name was not found there.
This makes indexing of separate modules totally independent and also
allows more complex resolutions with several import hops to be done.
Make function names more consistent

Renamed path2module and module2path to path_to_module and
module_to_path accordingly.