Pass interfaces instead of filenames to linters, so we only open files once

This should reduce the duplicate open/unmarshal/validate across linters. The duplicate validation is because there are some errors that are common to all linters.

• open file
• split along yaml separators (---) if necessary
• unmarshal to interface{} using sigs.k8s.io/yaml
• pass map[filename][entrynumber]interface{} to each linter