Change use of `NoDefault` to be private and maybe indicate it shouldn't be passed

[Dr-Irv]

NoDefault should never be used by users, but is useful when using stubtest to detect inconsistencies between the stubs and the source. See #848 (comment)

Should consider renaming NoDefault in the stubs to be private (e.g., _NoDefault), and maybe even something like _NoDefaultDoNotUse` .