module: Fix unsafe iteration

This fixes an issue when requesting module unload for
module-bluetooth-discover. When unloading the module, it also unloads
module-bluez4-discover and/or module-bluez5-discover, and that
invalidated the state variable that was used for iterating through the
modules idxset.

The pa_module.unload_requested flag could now otherwise be removed,
but it's still being (ab)used in the bluetooth modules.