Merge pull request #48 from tsloughter/master

use PRV_ERROR to wrap a provider error in {error, {?MODULE, ...}}
Print warning when deps are being skipped.

The case in mind here is due to conflicts, and tests have been added for
this.
Merge pull request #49 from rebar/skipped-deps-warnings

Print warning when deps are being skipped.
Fix error message to point to rebar3
Adding pkg deps tests and refactorings

- 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.
Merge pull request #60 from rebar/test-packages

Test packages
rewrite profiles
add 'as' higher order task
update readme for 'as'
don't include all */ebin dirs in code path