Overhaul codebase for modern rebar3

[Duncan McGreggor]

With the latest changes to the rebar.config, the ltest project has removed the conditions under which cyclic dependencies arise, in particular, with the latest LFE plugin for rebar3.

In the process of this, though, some cursory examination of project code revealed that much of the work various utility functions are doing have their history in a pre-mature rebar3 solution, during the time of lfetool.

Tasks:

• Does rebar3 provide utility functions for extracting metadata from beam files?
• Should we look at getting the same beam info, but from a different approach?
  • Can we instead get a list of all modules for a project (not reliable)
  • what about getting all beams in an app's ebin dir?: rebar_app_info:ebin_dir
• Should we continue using the behaviours as essentially marker interfaces
• Should we instead use a naming convention on file names?
• Remove all build tool/rebar specific code (move to rebar3 plugin)
  • create a default/constant for rebar3 dir, but don't use it in the beam-finder code
  • instead, offer it as a convenience for running project tests from a REPL