Pass correct from arg for resolved files

Pass correct 'from' parameter to postcss for files which resolved via `nodeResolve.sync`

Incorrect source path leads to wrong names which postcss-plugin-scope generates for example

Author: anru

file-system-loader.js

@@ -72,6 +72,8 @@ var FileSystemLoader = (function () {
         if (newPath[0] !== '.' && newPath[0] !== '/') {
           try {
             fileRelativePath = nodeResolve.sync(newPath, { basedir: rootRelativeDir });
+            // in this case we need to actualize rootRelativePath too
+            rootRelativePath = fileRelativePath;
           } catch (e) {}
         }