- 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

Change mock_pkg_resource to use rebar_test_utils:create_empty_app so
that an ebin directory is created which is a good expectation for a
package. Also revert the check in rebar_fetch:download_source to check
for an ebin directory before adding it to the code path for package
installs. If a package does not have an ebin directory it is probably
a good for an exception to be thrown.

- Adding tests for package deps
- Adding conflict/override warnings for package deps
- Adding cycle detection for packages
- Adding cycle detection for mixed packages+source
- Fixing internal dependency format of package resources when converted
  to rebar_app_info
- normalizing level-order prioritization to be based on lexicographical
  sort of app names rather than traversal order (which is undefined for
  package deps)
- Fixing tests for source deps for deep cycle detection
- Fixing bugs with source deps
- Relaxed version format checks for test utils

A lot of fixes are combined in there because I didn't want to commit
non-passing code, and many bugs were found when adding the second batch
of tests as part of the original effort.