Allow parent-relative and absolute plugin archive output paths

Allow Plugin SDK authors to write .objopackage archives to parent-relative or absolute output paths in plugin.json. Related plugins can then write directly into one shared output folder, for example "output": "../out/acme.graphics.png-1.0.0.objopackage", without a separate collection script. Relative archive paths are resolved from the directory containing plugin.json, regardless of the command's working directory.