code: function collectLayouts( pages ) {
        const followLayoutOnce = promiseOnce( followLayout );
        const followPageToLayouts = followEntryRefs( 'layouts', followLayoutOnce );
  
        return Promise.all( pages.map( followPageToLayouts ) )
           .then( flatten )
           .then( dedupe );
     }
docs: //////////////////////////////////////////////////////////////////////////////////////////////////////// 
  Finds layouts based on them being referenced in page areas.
  
  Example:
  
  collector.collectLayouts( pages )
  .then( layouts => {
  assert( Array.isArray( layouts ) );
  } );
  // => [ {
  //       refs: [ 'layout' ],
  //       name: 'layout',
  //       path: 'path/to/layout'
  //    }, ... ]
  
  @memberOf ArtifactCollector
  @param {Array} pages
  a list of page artifacts as returned by {@link ArtifactCollector#collectPages}
  @return {Promise<Array>}
  a promise for an array of meta-information about all layouts