This totally breaks compatibility with rebar2, and maybe it shouldn't
have.

Removed separate compilers
Resolves apps to build
Finds avail deps before pulling/building
Includes relx
Simplifies build commands

This reverts commit 07e2232847baa634b7d09bdb87dafda6c6bcc4a4, reversing
changes made to 37cf470ae9.

rebar.hrl is only meant to be used by src/*.

We have to rename ?FAIL to ?ABORT once we require at least R13B04 for
building rebar. Macros with different arity were not supported by the
compiler before 13B04.

The ability to overload macros was added in Erlang R13B04.
The second form was not used anyway.

This change adds support for executing ct test runs based on test
specificiations, which was missing previously. The rebar_ct module
now looks for any number of files with a name ending in `test.spec`
and if it finds one or more, passes these after the `-spec` argument
to ct_run instead of explicitly configuring the config, user config
and coverage config variables.

When no specifications are found, then the module behaves as it did
before this change, and both the ct1 and (new) ct2 integration tests
appear to show this is a backwards compatible patch.