lint: new tool for validation and normalization

[zombiezen]

I'd like to create a tool that takes in a catalog and outputs the same catalog, but has a series of optional transforms and validations. Examples:

• Sort resources and dependency lists (like in #7)
• Detect cycles (tracking in #5)
• Prune no-op resources by adding their direct dependencies as direct dependencies of their dependents
• Check file paths for absoluteness (and other preconditions specified in the schema docs)
• Move large blobs to separate segments (potentially enables optimizations in #11)