Move rabbitmq_prelaunch from deps/rabbit/apps to deps

[Why]
Building an Erlang release with this application under `apps` happened
to be problematic with Erlang.mk. Our use of deps and apps is not
exactly standard here anyway.

We also get issues with various make targets where `rabbitmq_prelaunch`
is being cleaned and not re-compiled, leading to "application not found"
errors.

[How]
So instead of fighting, move `rabbitmq_prelaunch` to the deps directory,
like all other applications.

This commit is not enough and will be followed with changes to Makefiles
and Bazel files. This is not done in this commit so the changes are
visible. Otherwise, they would be burried in the move.

Author: dumbbell