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.
The validator code contains (modified) fragments of the
local_plugins_archive_validator class copyrighted by Marina Glancy that
is part of the local_plugins plugin. It operates over an extracted
copy of the ZIP file.