don't special-case params named __ for historical positional params

Summary:
https://typing.readthedocs.io/en/latest/spec/historical.html#positional-only-parameters

The rule is:

> all parameters with names that begin but don’t end with __ are assumed to be positional-only

Strictly reading the rules, `__` ends with `__` so it should not be positional only. However, Pyre special-cases `__` to count as positional. Pyright doesn't have the special case, and Mypy doesn't implement this check from what I can tell.

Reviewed By: stroxler

Differential Revision: D60151159

fbshipit-source-id: 4ccb8133d6860ae844bc4a3c7071cb4a7605d3da

source/analysis/test/typeTest.ml

@@ -3704,8 +3704,8 @@ let test_parameter_create _ =
     (Type.Callable.CallableParamType.create
        [{ Type.Callable.CallableParamType.name = "__"; annotation = Type.integer; default = false }])
     [
-      Type.Callable.CallableParamType.PositionalOnly
-        { index = 0; annotation = Type.integer; default = false };
+      Type.Callable.CallableParamType.Named
+        { name = "__"; annotation = Type.integer; default = false };
     ]
 
 

source/analysis/type.ml

@@ -1470,9 +1470,8 @@ module Callable = struct
                 if
                   (not keyword_only)
                   && (not has_pep570_syntax)
-                  && (String.equal sanitized "__"
-                     || String.is_prefix sanitized ~prefix:"__"
-                        && not (String.is_suffix sanitized ~suffix:"__"))
+                  && String.is_prefix sanitized ~prefix:"__"
+                  && not (String.is_suffix sanitized ~suffix:"__")
                 then
                   index + 1, CallableParamType.PositionalOnly { index; annotation; default }
                 else

source/interprocedural_analyses/taint/accessPath.ml

@@ -191,9 +191,8 @@ let normalize_parameters parameters =
       let positional_only =
         (not seen_star)
         && (not has_pep570_syntax)
-        && (String.equal unqualified_name "__"
-           || String.is_prefix unqualified_name ~prefix:"__"
-              && not (String.is_suffix unqualified_name ~suffix:"__"))
+        && String.is_prefix unqualified_name ~prefix:"__"
+        && not (String.is_suffix unqualified_name ~suffix:"__")
       in
       ( position + 1,
         false,