Create linter for metadata

[mrunesson]

Create a linter for metadata.

It shall:

• verify necessary fields are included
• List what is processed or not
• If parts listed without data on disk
• If data on disk exist without matching parts.

[mrunesson]

This is generally done for the information currently used by the packer. Annotations regarding PII, contamination and propella are not used by the packer and therefore not checked by the linter. Though, there are issues to enable these (#106, #107, #108).