to `as` used to generate them

- fetch more complete refs for tags (may download more history than
  required, but it works)
- Fix comparison of tag distance by using lists everywhere

pre-purging before checking to facilitate long do chains

source files in the `compile` key of `module_info/1` so `rebar cover`
struggles locating source in some instances. in these cases a warning
is printed and crappy coverage info is written rather than none at all

`rebar.config` or via the `-c\--cover` flag given to the appropriate
task) from the `eunit` and `ct` tasks and add a `cover` task to
write coverage analysis to disk

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