rabbitmq_prelaunch: Run rabbit_prelaunch_conf:setup/1 earlier

This fixes the issue where applications' configuration is applied after
they are started (and they do not read their environment again after
startup). This is the case of applications such as `ra` or
`sysmon_handler`: they are dependencies of `rabbit` and the Erlang
application controller will start them before.

Now, the configuration is loaded during the first prelaunch phase by
rabbitmq_prelaunch, hopefully before those applications are started.

To permit this change, the code updating the `enabled_plugins_file` was
moved to its own module. This one can't be moved to the
rabbitmq_prelaunch application because it depends on `rabbit_plugins`.

While here, add a couple assertions by checking return values.

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