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.

When dialyzer_plt_warnings is false no warnings should be returned when
updating a PLT. Previously any callgraph warnings would be returned when
updating a PLT regardless of the get_warnings option.

Match erl_first_files properly against all files needing
compilation and make sure the order of erl_first_files as specified in
rebar.config is preserved (rebar/rebar#445)

Tests missing

Apps that are no longer used are not automatically deleted, but we tell
users it can be done. This is safer while we're not sure of the
correctness of these messages.

Error messages are added for transient dependencies and dependencies not
found.

- Many apps is supported through and through
- Not mentioning any app upgrades all apps
- Locks are refreshed on disk and tested as such after an upgrade

todo:
- relock stuff
- default to all apps needing upgrade
- more tests?
- pkgs?

- rename 'update' to 'upgrade' -- 'update' stands for package list
  updates, not dependencies upgrades
- extracting some deeply nested code

Only the most complex case is failing, where cross-dependencies would
need to be refetched as an update clears an app of its dependencies and
a different subtree should override it.

The problem with the current effort is handling of transitive dependency
upgrades and possible values.

Test that all the correct locks are set for an upgrade run.

Now to actually re-run the install deps and prove it works

commands

erts.app was introduced in R16B02 so read all beam files from the erts
ebin directory if erts.app file does not exist.