Add erts as an explicit dependency

[dumbbell]

Why

Horus relies on the availability of erts to initialize the list of modules to skip when it needs to decide if a call should be followed to extract a function or if the call should be preserved as is.

It already depends on kernel and stdlib for instance. I assumed that erts was implicit as I don't see how Erlang can work without it. It looks like if erts is not included explicitly in an Erlang release file or in the applications it bundles, it is excluded from the release (the erts Erlang application, not the actual runtime). This breaks Horus because it won't know about e.g. the erlang module and will try and fail to extract functions from that module.

See rabbitmq/khepri#212 and rabbitmq/khepri#222 for users who encountered this problem. A function extraction will fail with {horus_ex, module_not_found, #{module => erlang}} if erts is missing from a release.

How

It looks like a bug in the way releases are created because, to me, it makes no sense to not always include erts (like kernel and stdlib).

As a workaround, we should include erts in the dependencies of Horus.