Fix handling of imported source map.
Fix handling of imported source map.

Inlined @import files with existing source map results in relative path
as ‘sources’ value.

MichaelFBA/gulp-minify-css,zarko-tg/gulp-minify-css,sibrus/gulp-minify-css,mpath-development/gulp-minify-css,keremciu/gulp-minify-css,HaNdTriX/gulp-minify-css,LoicMahieu/gulp-minify-css,webconnex/gulp-minify-css