has the same outcome as `rebar3 foo && rebar3 bar && rebar3 baz`

fixes #191

See https://github.com/rebar/rebar/pull/458

The 'element(1, ..)' guard would always fail and cause crashes.

- Track level of packages properly, they're not level 0 anymore (this
  could yield an issue where a src dep takes precedence over a pkg dep)
- Proper stable sort of vertices in the digraph module
- PkgDeps no longer 'see themselves' when fetching and upgrading after
  locking themselves
- Pkg Locks are added to pkg deps rather than source deps
- Updating test cases to support pkg mocking on top of src mocking

fixes #175

fixes #172

Old one might have been a mistake; this one works fine.

- Make functions shorter
- Unnest some functions
- Split up and give names to major conditional branches

the process where ct is performed

during compile task before running subsequent tasks

arguments to ensure compile results as as intended

It is not convenient for everyone to handle unmatched_returns and
underspecs warnings. Extra default warnings are removed to match
dialyzer's behaviour.

Previously dialyzer would crash when not doing success typing analysis.