build(aio): add metadata aliases for directives, components and pipes (#19317)

This change will enable people to link to the API docs via their selectors
or names, as used in a template.

Since the selectors can be quite complex we are not able to get 100%
accuracy.

Closes #16787

aio/tools/transforms/angular-api-package/processors/addMetadataAliases.js

@@ -0,0 +1,40 @@
+/**
+ * @dgProcessor addMetadataAliases
+ *
+ * Directives and components can also be referenced by their selectors,
+ * and Pipes can be referenced by their name.
+ * So let's add each selector as an alias to this doc.
+ */
+module.exports = function addMetadataAliasesProcessor() {
+  return {
+    $runAfter: ['extractDecoratedClassesProcessor'],
+    $runBefore: ['computing-ids'],
+    $process: function(docs) {
+      docs.forEach(doc => {
+        switch(doc.docType) {
+        case 'directive':
+        case 'component':
+          doc.aliases = doc.aliases.concat(extractSelectors(doc[doc.docType + 'Options'].selector));
+          break;
+        case 'pipe':
+          if (doc.pipeOptions.name) {
+            doc.aliases = doc.aliases.concat(stripQuotes(doc.pipeOptions.name));
+          }
+          break;
+        }
+      });
+    }
+  };
+};
+
+function extractSelectors(selectors) {
+  if (selectors) {
+    return stripQuotes(selectors).split(',').map(selector => selector.replace(/^\W*([\w-]+)\W*$/, '$1'));
+  } else {
+    return [];
+  }
+}
+
+function stripQuotes(value) {
+  return (typeof(value) === 'string') ? value.trim().replace(/^(['"])(.*)\1$/, '$2') : value;
+}