We are also moving some internals into an internal header.
`HasAbslStringify` was not previously in an internal namespace but was intended
to be and has now been moved to an internal namespace. This is in adherence to
our compatibility guidelines which wave requirements for APIs within their
first 30 days of public release (See https://abseil.io/about/compatibility for
details).

PiperOrigin-RevId: 481190705
Change-Id: I4c0c348f269ea8d76ea3d4bd5a2c41cce475dc04