gh-106320: Remove private _PyMem API (#107187)

Move private _PyMem functions to the internal C API (pycore_pymem.h):

* _PyMem_GetCurrentAllocatorName()
* _PyMem_RawStrdup()
* _PyMem_RawWcsdup()
* _PyMem_Strdup()

No longer export these functions.

Move pymem_getallocatorsname() function from _testcapi to _testinternalcapi,
since the API moved to the internal C API.