code: function getAliasesForExtensions( { extensionsDirectory } ) {
  	const extensionsNames = fs
  		.readdirSync( extensionsDirectory )
  		.filter( filename => filename.indexOf( '.' ) === -1 ); // heuristic for finding directories
  
  	const aliasesMap = {};
  	extensionsNames.forEach(
  		extensionName =>
  			( aliasesMap[ extensionName ] = path.join( extensionsDirectory, extensionName ) )
  	);
  	return aliasesMap;
  }
docs: This function scans the /client/extensions directory in order to generate a map that looks like this:
  {
  sensei: 'absolute/path/to/wp-calypso/client/extensions/sensei',
  woocommerce: 'absolute/path/to/wp-calypso/client/extensions/woocommerce',
  ....
  }
  
  Providing webpack with these aliases instead of telling it to scan client/extensions for every
  module resolution speeds up builds significantly.
  @returns {Object} a mapping of extension name to path