Move package IDs from UUID to slug

[bates64]

Why?

• Makes git branch names understandable
• Makes asset directory names understandable
• Having a consistent slug is nice for URLs for package registry when that comes about

Implementation:

• Migration routine for existing packages
  • All(?) UUIDs are valid slugs, so, if we read an ID that looks like a UUID, warn. See below
• Emit warning if manifest id is not a slugged version of name? i.e. convert to kebab lower case
  • May want to reserve this check for a future merlon publish command