HEAD of Alpine SecDB now uses one consistent schema for all of their
vulnerabilities, so the logic around parsing different versions can now
be removed. This change also crawls the directory structure to parse all
files due to the addition of community.yaml tracking community Alpine
Linux packages.
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.
