build(docs-infra): support doc aliases via @alias dgeni tag (#29673)

Now, one can add an `@alias` tag to API docs, which tells dgeni that this
API element (usually a `const`) is really just an alias for some API element
defined elsewhere.

Dgeni will then look up this API element and copy over the properties from
the alias to the current doc.

For example, we would like to privately export an Enum from `@angular/core`
but then publicly export this from `@angular/common`:

**packages/core/private_exports.ts**

```ts
/**
 * Description of this document.
 */
export enum ɵSomeEnum { ... }
```

**packages/common/public_api.ts**

```ts
import {ɵSomeEnum} from '@angular/core';

 /**
 * @alias core/ɵSomeEnum
 */
export const SomeEnum = ɵSomeEnum;
```

In the generated docs there will be a page for `common/SomeEnum`, which
will be rendered as an enum, rather than a const, showing the description
extracted from the `core/ɵSomeEnum`.

---

The implementation of this feature required some refactoring of the other
processing:

1. Previously `ɵ` prefixed exports were not even considered.
2. Due to 1. some processors needed to have guards added to ignore such
   private exports (`addMetadataAliases` and `checkContentRules`).
3. The processing of package pages had to be reworked (and split) so that
   it picked up the aliased export docs after their alias proeprties had
   been copied.

See FW-1207, FW-632, #29249

PR Close #29673

aio/tools/transforms/angular-api-package/index.js

@@ -32,9 +32,11 @@ module.exports = new Package('angular-api', [basePackage, typeScriptPackage])
   .processor(require('./processors/simplifyMemberAnchors'))
   .processor(require('./processors/computeStability'))
   .processor(require('./processors/removeInjectableConstructors'))
+  .processor(require('./processors/collectPackageContentDocs'))
   .processor(require('./processors/processPackages'))
   .processor(require('./processors/processNgModuleDocs'))
   .processor(require('./processors/fixupRealProjectRelativePath'))
+  .processor(require('./processors/processAliasDocs'))
 
 
   /**
@@ -72,7 +74,7 @@ module.exports = new Package('angular-api', [basePackage, typeScriptPackage])
 
     // API files are typescript
     readTypeScriptModules.basePath = API_SOURCE_PATH;
-    readTypeScriptModules.ignoreExportsMatching = [/^[_ɵ]|^VERSION$/];
+    readTypeScriptModules.ignoreExportsMatching = [/^[_]|^VERSION$/];
     readTypeScriptModules.hidePrivateMembers = true;
 
     // NOTE: This list should be in sync with tools/public_api_guard/BUILD.bazel