Keep folder structure with concat() in gulp

Folder structure:

project
|
+-coffee
| |
| +-main.coffee
| |
| +-testDir
| | |
| | +-models.coffee
| | |
| | +-views.coffee
| |
| +-anotherDir
| | |
| | +-routes.coffee
| | |
| | +-views.coffee
| | |
| | +-modules.coffee
| |
| +- etc...
| 
+-www

The idea is to keep the folder structure from the coffee/ directory when writing files to the www/ directory. There can be an arbitrary number of subfolders in coffee/. All .coffee files from each folder should be concatened into a modules.js file:

www
|
+-modules.js
|
+-testDir
| |
| +-modules.js
|
+-anotherDir
| |
| +-modules.js
|
+- etc...

I currently have this gulp task:

gulp.task('coffee', function() {
    gulp.src('./coffee/**/*.coffee', {base: './coffee/'})
        .pipe(coffee({bare: true}).on('error', gutil.log))
        .pipe(uglify())
        // .pipe(concat('modules.js'))
        .pipe(gulp.dest('./www'))
});

Without the concat() the files are placed into the correct subfolders (but they're not concatened). With the concat() all files are concatened into a single modules.js file:

www
|
+-modules.js

How I can realize this correctly?

Here's a solution using gulp-flatmap:

var flatmap = require('gulp-flatmap');

gulp.task('coffee', function() {
  return gulp.src('./coffee/{*,}/', {base:'./coffee'})
    .pipe(flatmap(function(stream, dir) {
       return gulp.src(dir.path + '/*.coffee')
         .pipe(coffee({bare: true}).on('error', gutil.log))
         .pipe(uglify())
         .pipe(concat('modules.js'))
         .pipe(gulp.dest('./www/' + path.relative(dir.base, dir.path)))
     })) 
});

This first puts the coffee/ directory and all of its direct subdirectories in a stream. Each of those directories then gets mapped to a new stream that concatenates all .coffee files in the respective directory. Finally the appropriate destination folder for each resulting modules.js file is determined by using path.relative().