bpo-45395: Make custom frozen modules additions instead of replacements.

[ericsnowcurrently]

Currently custom modules (the array set on PyImport_FrozenModules) replace all the frozen stdlib modules. That can be problematic and is unlikely to be what the user wants. This change treats the custom frozen modules as additions instead. They take precedence over all other frozen modules except for those needed to bootstrap the import system. If the "code" field of an entry in the custom array is NULL then that frozen module is treated as disabled, which allows a custom entry to disable a frozen stdlib module.

This change allows us to get rid of is_essential_frozen_module() and simplifies the logic for which frozen modules should be ignored.

https://bugs.python.org/issue45395

[gvanrossum]

Same comment as for #28776 -- do we have any examples of apps that actually use this feature? Would they benefit from this change?

[ericsnowcurrently]

I don't have examples, other than that PyImport_FrozenModules is what Tools/freeze/makefreeze.py uses. (Given @malemburg's requests elsewhere, Tools/freeze appears to be used in the wild.) The division between the kinds of frozen modules I introduce here allows us to simplify some of the logic in import.c, so I consider it worth doing regardless of any benefit to projects that set custom frozen modules. The only catch is that the existence of PyImport_FrozenModules complicates matters if we leave it as an override instead of making it additions.

[gvanrossum]

Aha, so then we should just make sure that Tools/freeze still works. :-)