Fix builtin import issues

jedi/api/__init__.py

@@ -185,7 +185,9 @@ class Script(object):
         module, = try_to_merge_with_stub(
             self._evaluator, None, module.string_names, ContextSet([module])
         )
-        self._evaluator.module_cache.add(names, ContextSet([module]))
+        if names[0] not in ('builtins', '__builtin__', 'typing'):
+            # These modules are essential for Jedi, so don't overwrite them.
+            self._evaluator.module_cache.add(names, ContextSet([module]))
         return module
 
     def __repr__(self):

jedi/evaluate/__init__.py

@@ -138,6 +138,12 @@ class Evaluator(object):
             ),
             self,
         )
+        # TODO remove this `if`
+        if project is not None:
+            # These imports are essential for Jedi and should therefore always
+            # be present in the module cache.
+            self.builtins_module
+            self.typing_module
 
     def import_module(self, import_names, parent_module_context=None,
                       sys_path=None, load_stub=True):