[3.14] gh-132775: Make _PyXI_session Opaque (gh-134522)

This is mostly a refactor to clean things up a bit, most notably the "XI namespace" code.

Making the session opaque requires adding the following internal-only functions:

* _PyXI_NewSession()
* _PyXI_FreeSession()
* _PyXI_GetMainNamespace()

(cherry picked from commit 4a4ac3ab4d, gh-134452)

Co-authored-by: Eric Snow <ericsnowcurrently@gmail.com>