fix build source ordering

Apparently this is something mypy is sensitive to. Thanks mypy primer!

Author: hauntsaninja

mypy/find_sources.py

@@ -47,16 +47,18 @@ def create_source_list(paths: Sequence[str], options: Options,
     return sources
 
 
-def keyfunc(name: str) -> Tuple[int, str]:
+def keyfunc(name: str) -> Tuple[bool, int, str]:
     """Determines sort order for directory listing.
 
-    The desirable property is foo < foo.pyi < foo.py.
+    The desirable propertes are:
+    1) foo < foo.pyi < foo.py
+    2) __init__.py[i] < foo
     """
     base, suffix = os.path.splitext(name)
     for i, ext in enumerate(PY_EXTENSIONS):
         if suffix == ext:
-            return (i, base)
-    return (-1, name)
+            return (base != "__init__", i, base)
+    return (base != "__init__", -1, name)
 
 
 def normalise_package_base(root: str) -> str: