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.

The tests were waiting for the test group to finish before
they could continue with the next test group. Now "core"
and "all" targets directly depend on individual test cases,
allowing parallel Make to get to the next tests quicker and
removing 1/3rd of the total run time.

  make check -j8 -k  5790,16s user 1207,08s system 627% cpu 18:35,49 total
  make check -j8 -k  6250,13s user 1326,77s system 972% cpu 12:59,16 total

Somehow the develop and master branches of LFE have different
commit hashes and because LFE switched its default branch to
develop the hash we use for testing became incorrect. To fix
I just switched to the develop hash of the equivalent commit.

This is a waste of time and if they were to fail, this does not serve
the Erlang.mk testsuite.

And use this in the tests to fetch Rebar only once and then
clone from the local repository. This should speed up tests
a little.