WP-CLI has organically evolved into a monolithic codebase that can now be abstracted to a set of discrete packages. In order to better support a future with lots of disparate WP-CLI features, let's split internal commands into a series of distinct packages, beginning with:

• wp (cache|transient) * https://github.com/wp-cli/cache-command
• wp checksum https://github.com/wp-cli/checksum-command
• wp config * https://github.com/wp-cli/config-command
• wp core * https://github.com/wp-cli/core-command
• wp cron * https://github.com/wp-cli/cron-command
• wp db * https://github.com/wp-cli/db-command
• wp (eval|eval-file) https://github.com/wp-cli/eval-command
• wp export https://github.com/wp-cli/export-command
• wp (option|post|comment|user|term|site) * https://github.com/wp-cli/entity-command
• wp import https://github.com/wp-cli/import-command
• wp language https://github.com/wp-cli/language-command
• wp media * https://github.com/wp-cli/media-command
• wp package * https://github.com/wp-cli/package-command
• wp (plugin|theme) * https://github.com/wp-cli/extension-command
• wp rewrite https://github.com/wp-cli/rewrite-command
• wp (role|cap) * https://github.com/wp-cli/role-command
• wp scaffold * https://github.com/wp-cli/scaffold-command
• wp search-replace https://github.com/wp-cli/search-replace-command
• wp server https://github.com/wp-cli/server-command
• wp shell https://github.com/wp-cli/shell-command
• wp super-admin * https://github.com/wp-cli/super-admin-command
• wp (widget|sidebar) * https://github.com/wp-cli/widget-command

When we split the packages, we should:

• Preserve Git history
• Mirror Travis' test suite matrix in the package

Previously: #3652 (comment)