Main Clair logic is changed in worker, updater, notifier for better adapting
ancestry schema. Extensions are updated with the new model and feature lister
and namespace detector drivers are able to specify the specific listers and
detectors used to process layer's content. InRange and GetFixedIn interfaces
are added to Version format for adapting ranged affected features and next
available fixed in in the future. Tests for worker, updater and extensions
are fixed.
This removes the `types` package instead moving the contents to the
top-level clair package.
This change also renames the `Priority` type to `Severity` in order to
reduce confusion.
This change also removes the IsValid method and replaces it with a safe
constructor to avoid the creation of invalid values.
Many docstrings were tweaked in the making of this commit.
