pipewire: module-zeroconf-discover: handle module destroy events

module-zeroconf-discover loads module-pulse-tunnel for each entity
exposed on the network. Previously, however, the destroy event
of the loaded pulse-tunnel modules were not handled.

This resulted in a use-after-free because both `pw_context_destroy()`
and `module-zeroconf-discover.c:impl_free()` tried to destroy
the pulse-tunnel modules. The reason for that is that since
1de16afc the modules
are prepended to the module list of the context, not appended,
therefore modules are destroyed in LIFO order, thus the pulse-tunnel
modules were destroyed before the zeroconf-discover module that
loaded them.

Fix that by handling the destroy event of the loaded pulse-tunnel
modules.

Fixes #1653