This commit switches the relx dependency to branch 4.0.0. It is the
first iteration of integration with relx 4.0 which adds many changes
to how relx works and how the two integrate.

rebar_relx module now creates a relx state from the relx configuration
and pass it along with all the OTP applications rebar3 knows about
which may be used in the release. This allows relx to not have
to duplicate the effort of application discovery like before.

Since this is intended to be released only when OTP-23 is released
the testing of OTP-18 has been removed.

This replaces all deprecated function usage by alternative ones based on
a version switch enacted at compile time, preventing all warnings.

This will likely introduce some possible runtime errors in using a
Rebar3 compiled on OTP-20 or OTP-21 back in versions 19 and earlier, but
we can't really work around that.

A bunch of dependencies have been updated to support OTP-21 without
warnings as well.

This mostly moves a lists:reverse/1 which ensures that profile overlays
are run first but keeps the order of overlays otherwise.

This is so that some of them can be enforced
(such as warnings_as_errors) on relx's task like
generating a release or a release upgrade.

Relx config need new values at the end of the list and
profile additions prepended. So now in apply_profile in
the case it is a relx config we prepend the config list.
This does, however, mean that a profile release can not
'extend' a release defined in the default profile since
the one being extended would have its definition after
the profile release and thus relx would not be able to
use its values for extending.

fixes #872