Remove use of dynamic require to facilitate bundling (#960)

lib/index.js

@@ -1,17 +1,13 @@
 'use strict';
 
 const Sharp = require('./constructor');
-[
+require('./input')(Sharp);
-  'input',
+require('./resize')(Sharp);
-  'resize',
+require('./composite')(Sharp);
-  'composite',
+require('./operation')(Sharp);
-  'operation',
+require('./colour')(Sharp);
-  'colour',
+require('./channel')(Sharp);
-  'channel',
+require('./output')(Sharp);
-  'output',
+require('./utility')(Sharp);
-  'utility'
-].forEach(function (decorator) {
-  require('./' + decorator)(Sharp);
-});
 
 module.exports = Sharp;