feat(build): Make cleanup, license [sic] tasks invoke their prerequisites

Turns out they don't have any, so this commit just classifies
their gulp targets according to the established scheme.

gulpfile.js

@@ -37,10 +37,12 @@ module.exports = {
   publishBeta: releaseTasks.publishBeta,
   gitUpdateGithubPages: gitTasks.updateGithubPages,
 
-  // Manually-invokable targets that invoke prerequisites.
+  // Manually-invokable targets, with prequisites where required.
   prepare: buildTasks.prepare,
   format: buildTasks.format,
   generate: buildTasks.generate,
+  sortRequires: cleanupTasks.sortRequires,
+  checkLicenses: licenseTasks.checkLicenses,
   clean: gulp.parallel(buildTasks.cleanBuildDir, packageTasks.cleanReleaseDir),
   buildAdvancedCompilationTest: buildTasks.buildAdvancedCompilationTest,
   gitCreateRC: gitTasks.createRC,
@@ -57,7 +59,5 @@ module.exports = {
   // TBD.
   deployDemos: appengineTasks.deployDemos,
   deployDemosBeta: appengineTasks.deployDemosBeta,
-  checkLicenses: licenseTasks.checkLicenses,
   prepareDemos: appengineTasks.prepareDemos,
-  sortRequires: cleanupTasks.sortRequires,
 };

scripts/gulpfiles/cleanup_tasks.js

@@ -81,5 +81,6 @@ function sortRequires() {
 };
 
 module.exports = {
+  // Manually-invokable targets, with prequisites where required.
   sortRequires: sortRequires
 };

scripts/gulpfiles/license_tasks.js

@@ -17,5 +17,6 @@ function checkLicenses() {
 };
 
 module.exports = {
+  // Manually-invokable targets, with prequisites where required.
   checkLicenses: checkLicenses
 };