Better package names in pkg

[rbehjati]

Under the pkg folder we currently have the following packages:

• amber: containing claims and endorsements structs
• intoto: Containing intoto statement and slsa provenance structs
• types: containing generic functionality for parsing provenances

This issue suggests renaming the packages amber and types.

amber is a legacy package name that we no longer use anywhere in the documentation. This package can be renamed to claims. Alternatively, this package can be split into two: claims and endorsements instead of having the separation only at the file-level.

types is a very generic name. This could be renamed to validation, or provenancevalidation but that is too long for a package name.

@tiziano88 @mariaschett What do you think? If you think this is a good change, I'll do it and then create a v1 release.