gh-151950: Fix Sphinx reference warnings in Doc/library/importlib.rst - #157610

Eleven "reference target not found" warnings come from Doc/library/importlib.rst (the full list is in the issue), with four separate causes.

Most of them are references written without a module -- Loader.create_module, Loader.exec_module, InspectLoader.is_package and import_module -- used from paragraphs that sit in the importlib.machinery and importlib.util sections, where importlib.abc and importlib are not the current module. The file already spells these out elsewhere, as :meth:`spec.loader.create_module <importlib.abc.Loader.create_module>``` in `module_from_spec()` and as :func:`importlib.import_module``` further down, so this uses the same qualified forms; the rendered text stays as it was.

cache_tag is described inside the sys.implementation entry in Doc/library/sys.rst and has no target of its own. Doc/whatsnew/3.12.rst already links it as ``:attr:`sys.implementation.cache_tag <sys.implementation>```, which is what the two references here now do.

get_resource_reader has not been documented since gh-143513 removed the importlib.abc.ResourceReader section. That commit converted the references left behind in Doc/whatsnew/3.7.rst and Misc/NEWS.d to the suppressed ``:meth:`!...``` form, but missed these two.

The last one, get_tag, is imp.get_tag(), removed in 3.12. The same sentence already points at sys.implementation.cache_tag, which replaced it, so the dead reference is dropped rather than retargeted. That is the only place where the rendered text changes.

I checked with a nit-picky docs build (make -C Doc SPHINXOPTS="--nitpicky --warning-file ..." html) against a clean build of main: the eleven warnings are gone, no new ones appear anywhere in Doc/, and Doc/tools/check-warnings.py --fail-if-regression --fail-if-improved --fail-if-new-news-nit exits 0. Doc/library/importlib.rst therefore comes out of Doc/tools/.nitignore.

Found and fixed with AI assistance. I reproduced the eleven warnings against a clean build of main, traced each one to its cause myself, and checked that the four reference forms used here are the ones already in the tree; the docs build above is my own verification.

Sub-issue of #151940.

• Issue: Fix Sphinx reference warnings in various Doc/library/ files #151950