Add package searching through their metadata

[doom]

• Add a nest search subcommand
• Find packages with advanced search criteria (name, description, content, etc...)
• Support both positive (--with-name=libc) or negative (--without-name=libc) filtering
• Add a nest list subcommand
• List only installed packages (only the statically-required ones, or all of them using --with-deps)
• Can be implemented in terms of nest search