Allow each subcommand to be run before WP is loaded.

WP isn't needed for package management.
Abstraction of `wp_list_pluck()` so we can run this without WordPress.
Package management requires the Composer PHP library.
WP-CLI community commands need to require WP-CLI as a dependency
a275bcb75146547964e6d9347652434e3668da53 in less words
Use the WP-CLI package index as our directory.
A subcommand for browsing existing community packages