Input source maps: source filenames should be relative

Should the `sources` in `concatenated.css.map` should be relative to the map file? https://github.com/plumberjs/plumber-less/blob/11c837509748e3285ee5aac3b358319fc0b24278/test/fixtures/concatenated.less.map

This is a mistake I identified when implementing plumber-rework: https://github.com/reworkcss/css/issues/52#issuecomment-55431673.