bpo-45396: Always import custom frozen modules.

[ericsnowcurrently]

If a Python build has a custom array set to PyImport_FrozenModules (see Python/frozen.c) then that array is used instead of the default one. In that case the whole point is that the custom frozen modules should be used. So in this change we ignore the PyConfig.use_frozen_modules setting if there are custom frozen modules. We also change the "-X frozen_modules" CLI option to "-X frozen_stdlib" to reflect the more specific context.

https://bugs.python.org/issue45396

[ericsnowcurrently]

@brettcannon, this is the result of #28633 (comment).

[gvanrossum]

Can we have a bit more discussion about this before this is merged? Do you have an example of a real-word app that sets PyImport_FrozenModules? Depending on why that app is doing that, it might or might not make sense to allow disabling loading frozen modules. For example, if they use it to shadow .py modules that they have elsewhere in their package, it might still make sense to disable the frozen modules. Just because Brett initially misunderstood the meaning of -X frozen_modules doesn't mean that his interpretation is the most useful.

[ericsnowcurrently]

Setting PyImport_FrozenModules to define custom frozen modules is part of the API. Setting the data of an entry to NULL isn't so obviously a part of the API. However, the code in import.c definitely already supports this mechanism. So my intention was to preserve the existing behavior, whether or not folks are using it.

Regardless, the point is probably moot. If gh-28778 is merged (which I expect) then this PR is unnecessary. The only reason I hadn't closed this one is on the chance that the other PR ends up closed. In the meantime I'm closing this one.