bpo-45211: Remember the stdlib dir during startup. (gh-28586)

During runtime startup we figure out the stdlib dir but currently throw that information away. This change preserves it and exposes it via PyConfig.stdlib_dir, _Py_GetStdlibDir(), and sys._stdlib_dir.

https://bugs.python.org/issue45211

Include/internal/pycore_pathconfig.h

@@ -13,6 +13,7 @@ typedef struct _PyPathConfig {
     wchar_t *program_full_path;
     wchar_t *prefix;
     wchar_t *exec_prefix;
+    wchar_t *stdlib_dir;
     /* Set by Py_SetPath(), or computed by _PyConfig_InitPathConfig() */
     wchar_t *module_search_path;
     /* Python program name */