font.save("foo.glyphspackage") silently saves a single-file-format Glyphs file

....which the user then can't open. We should either give a warning or better, actually save in .glyphspackage format.

Start by reproducing the behavior through font.save("foo.glyphspackage") and trace the save-format handling in the library. Determine whether the intended outcome is a warning or a valid .glyphspackage save, then verify that the resulting file can be opened.

Written by the indexing model from the issue text.