repo rework: Remove need for repos to be self-aware

* See #12917 for details.
* Squashed to one commit to make revert easy if we
  run into any issues.
* pkgman is now pre-attached to the 'current' repo
  version within nightly images so they can be updated
  by default.
* This shouldn't impact us keeping older sets of package
  versions by commit hash for building older hrevs.
* There are XXX stubs with "Kill me". These will need
  to be dropped after users are given sufficent time to
  upgrade. We're dropping a previously required field (url)
  so making this a slowish roll out.
* Makes the repos a lot less restrictive which should
  help PM package building automation be a bit easier.
* Once this stuff smooths out, we'll add UUID's to the
  repo definitions for duplicate repo detection.

build/scripts/build_haiku_image

@@ -7,7 +7,7 @@ set -o errexit
 # tmpDir
 # addBuildCompatibilityLibDir
 # systemPackages - lists of the hpkg packages copied/updated
-# repositories - all repository files
+# repositories - all repository configuration files
 # downloadDir
 # The following are only for image types:
 # installDir