[DLST 473] New validation rules for manifest file

[goldsmithb]

Jira Ticket

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This PR adds a new validation rule and alters another:

• "3.1 Files in manifest exist" is changed to "3.1 Files in manifest exist in the data/ directory"
  • This ensures that the package is structured properly
• A new validation rule "3.2 No metadata Files in the manifest" was created
  • This explicitly checks that metadata files like the assets and items csv files are not included in the manifest file

[elohanlon]

🎉