build: modularize the gulp file to be easier to maintain (#14259)

This is a precursor to bringing in some of the gulp tasks from angular.io,
which could lead to the gulpfile becoming unwieldy.

tools/gulp-tasks/check-cycle.js

@@ -0,0 +1,16 @@
+module.exports = (gulp) => (done) => {
+  const madge = require('madge');
+
+  const dependencyObject = madge(['dist/all/'], {
+    format: 'cjs',
+    extensions: ['.js'],
+    onParseFile: function(data) { data.src = data.src.replace(/\/\* circular \*\//g, '//'); }
+  });
+  const circularDependencies = dependencyObject.circular().getArray();
+  if (circularDependencies.length > 0) {
+    console.log('Found circular dependencies!');
+    console.log(circularDependencies);
+    process.exit(1);
+  }
+  done();
+};