Plugins' files from before Mods/.reskate_plugin_data were still in Mods/.reskate/data, and the
launcher's merge deletes Mods/.reskate before the game ever looks for a plugin's folder: a saved
Deathrace route went at the next launch that rebuilt the merge. mods::keep_plugin_data moves each
plugin's old folder to Mods/.reskate_plugin_data first (whole, or the files a folder there lacks,
its own kept), at the start of every merge; the game still runs it once for a merge that kept its
patch. Checked in the build stamp test.