Implement missing schema validations

[paupino]

• Introduces unit tests for package.rs actions (defaults, graphs, validation)
• Implements simple "missing schema" validations with option to add further reference type errors.
• Fixes hard coded issue where the public schema was being assumed for defaults.
• Updated and ran rustfmt somewhat regrettably as it has introduced a lot of noise.

Let me know if you want me to roll back one commit to better indicate changes. I'll comment on the items to look out for.