Fix calculate_dotted_path_from_sys_path. It was broken beyond stupid.

jedi/evaluate/sys_path.py

@@ -197,31 +197,38 @@ def _get_buildout_script_paths(search_path):
             continue
 
 
-def dotted_path_in_sys_path(sys_path, module_path):
+def calculate_dotted_path_from_sys_path(sys_path, module_path):
     """
     Returns the dotted path inside a sys.path as a list of names.
+
+    This function is supposed to be a backup plan in case there's no idea where
+    a file is lying.
     """
     # First remove the suffix.
     for suffix in all_suffixes():
         if module_path.endswith(suffix):
             module_path = module_path[:-len(suffix)]
-        break
+            break
     else:
         # There should always be a suffix in a valid Python file on the path.
         return None
 
-    if module_path.startswith(os.path.sep):
-        # The paths in sys.path most of the times don't end with a slash.
-        module_path = module_path[1:]
+    if module_path.endswith('__init__'):
+        module_path = module_path[:-len('__init__') - 1]
+
+    if module_path.endswith(os.path.sep):
+        # The paths in sys.path may end with a slash.
+        module_path = module_path[:-1]
 
     for p in sys_path:
         if module_path.startswith(p):
             rest = module_path[len(p):]
+            if rest.startswith(os.path.sep) or rest.startswith('/'):
+                rest = rest[1:]
             if rest:
                 split = rest.split(os.path.sep)
                 for string in split:
                     if not string or '.' in string:
                         return None
-                return split
-
+                return tuple(split)
     return None