Most of the functionality provided by this tool (typically the
validation and actual deployment of the plugin package) has been moved
to the core level. So this is becoming just a thin wrapper and user
interface for installing new plugins via the administration UI.
Also fixes MDL-49600 as we no longer keep the unzipped contents of the
packages in the persistent temp directories.
The script validate.php expects a ZIP file stored in a temporary
location. It extracts the ZIP (optionally renaming the root directory)
and executes the validator. Then it renders the validator log messages
and continue buttons.