Replace package database with independently installed package repositories

- Add support for fetching and installing from package repositories containing package URLs and metadata
- Support multiple package repository sources
- Add new repo command
- Remove submit command
- Enhance describe and search commands
- Fix and improve help text for new and adjusted commands
- Fix bundle install exit status
- Refer to plugins as "plugins" instead of the general term "packages"
- Code clarity improvements
- Put plugin/theme distinction into package metadata