Provide a command line interface for the plugin manager features

Currently, the upgrade via command line fails if there are unsatisfied plugin dependencies. Also, there is no CLI alternative to all the nice new features introduced in MDL-49329 such as ability to update all installed plugins etc.

So, this is a proposal to have a new admin/cli/plugman.php tool with wide range of accepted parameters to perform operations like

• List all installed plugins in a requested format, e.g. php plugman.php --list --fields=name,versiondb,rootdir --separator=';'
• Search for plugins available in the plugins directory
• Install/upgrade plugins
• Resolve available missing dependencies

etc.