Also expose is_singleton_depset on py_internal.

This will be necessary to implement `py_runtime` in regular Starlark, so
expose it now so it's available later.

Work towards #15897

PiperOrigin-RevId: 485180942
Change-Id: I9b79b9d3938de80307a94b6db37569c0a577f8d6

src/main/starlark/builtins_bzl/common/python/py_internal.bzl

@@ -18,5 +18,9 @@ Various builtin Starlark defined objects exposed for non-builtin Starlark.
 These may change at any time and are closely coupled to the rule implementation.
 """
 
+_py_builtins = _builtins.internal.py_builtins
+
 # This replaces the Java-defined name using exports.bzl toplevels mapping.
-py_internal = struct()
+py_internal = struct(
+    is_singleton_depset = _py_builtins.is_singleton_depset,
+)