Move code from decimal.pyi to _decimal.pyi (#7640)

* Move code from `decimal.pyi` to `_decimal.pyi`

The stub previously did `from decimal import *` in `_decimal.pyi`, but the opposite happens at runtime.

stdlib/_pydecimal.pyi

@@ -2,7 +2,7 @@ import sys
 
 # This is a slight lie, the implementations aren't exactly identical
 # However, in all likelihood, the differences are inconsequential
-from decimal import *
+from _decimal import *
 
 if sys.version_info >= (3, 7):
     __all__ = [