moving get_models_from_apps helper
moved checks of primary_key and unique of FileField
reorganized invalid_models/tests.py
removed unnecessary indirection through ValueError
improved ambiguously phrased error message
added hint to error about using '?' marker with other fields
small style improvement
labeled contenttypes checks with 'models' tag
reorganized 'check' package
renamed django/core/checks/{default_checks.py -> model_checks.py}
made Debug, Info, ..., Critical classes instead of partial