PR-29419 solves the issue in #45717 of not giving a link for _* modules with no doc.  It does not solve this issue of giving the proper link for module that need non-standard links, which typically need a '#' suffix, as in '.../library/2to3.html#module-lib2to3'.  I suggest to continue calculating standard links as done now as the precalculation needs much space for a trivial time saving.  Restrict the new dict to non-standard links.  They can be calculated, with the needed suffixes, as they are now for the index.  The link logic would be to try the lookup first and if module name does not start with '_', calculate the link.

I will add an idlelib section to idle.rst so that 'idlelib' appears in the module index and get entered into the exceptions dict.

I don't see how PR-29459 get linked here as it has no reference to this issue.  So I will ignore it at least for know.