name conflict with builtin modules

When a library uses module names that conflicts with builtins, such as `_ast.py`, the import logic in https://github.com/davidhalter/jedi/blob/master/jedi/inference/compiled/subprocess/functions.py#L139 ends up using `importlib.machinery.BuiltinImporter` as it appears before `importlib.machinery.PathFinder` in `sys.meta_path`.

Start at jedi/inference/compiled/subprocess/functions.py around line 139 and inspect how importlib.machinery.BuiltinImporter and PathFinder are used through sys.meta_path. Reproduce the _ast.py name conflict and verify that a library module is selected instead of the builtin when both names collide.

Written by the indexing model from the issue text.