This commit also includes a way to completely disable Eunit
as that is generally desirable for Elixir-only projects.

This includes the SSL security fix for OTP-25+.

This should both be more solid and more secure.

Erlang.mk will no longer feature packages out of the box,
except for the applications that we implement plugins for
(such as erlydtl, proper or relx) and the projects I work
on that are also used to test Erlang.mk (cowboy and friends).

This is a breaking change. In most cases the fix is to add
a full dep_* line for the dependencies that were used as
packages before.

We now provide the version as a tagged tuple indicating
whether this is a branch, tag or ref. This should help
rebar automatically upgrade dependencies, and remove
an annoying warning when rebar is used.

In order to detect the type of version the dependencies
must be available, therefore the rebar.config target now
depends on the deps target.

To test this the Cowboy package version had to be changed
from 1.0.4 to master. This is a breaking change, but I do
not expect too many people to still use 1.0.4.

These are all fairly low risk because they haven't compiled
for a while. A small number seem maintained in a fork and
have a hex.pm entry that is maintained. The others are likely
not used by anyone or only by users stuck on very old Erlang
versions.