gh-106320: Remove private _PyTraceMalloc C API functions (#106324)

* Remove private _PyTraceMalloc C API functions: move them to the
  internal C API.
* Don't export most of these functions anymore, but still export
  _PyTraceMalloc_GetTraceback() used by tests.
* Rename Include/tracemalloc.h to Include/cpython/tracemalloc.h