fix(@angular/pwa): support using default index option when not present

The `@angular/pwa` add schematic now attempts to discover a default index
option value if a configuration usage path within the build target is
possible. The default value as per the `application` build system is
`<project_source_root>/index.html`.

(cherry picked from commit 8129483)

Author: clydin

packages/angular/pwa/pwa/index.ts

@@ -110,9 +110,12 @@ export default function (options: PwaOptions): Rule {
 
     // Find all index.html files in build targets
     const indexFiles = new Set<string>();
+    let checkForDefaultIndex = false;
     for (const target of buildTargets) {
       if (typeof target.options?.index === 'string') {
         indexFiles.add(target.options.index);
+      } else if (target.options?.index === undefined) {
+        checkForDefaultIndex = true;
       }
 
       if (!target.configurations) {
@@ -122,13 +125,23 @@ export default function (options: PwaOptions): Rule {
       for (const options of Object.values(target.configurations)) {
         if (typeof options?.index === 'string') {
           indexFiles.add(options.index);
+        } else if (options?.index === undefined) {
+          checkForDefaultIndex = true;
         }
       }
     }
 
     // Setup sources for the assets files to add to the project
     const sourcePath = project.sourceRoot ?? posix.join(project.root, 'src');
 
+    // Check for a default index file if a configuration path allows for a default usage
+    if (checkForDefaultIndex) {
+      const defaultIndexFile = posix.join(sourcePath, 'index.html');
+      if (host.exists(defaultIndexFile)) {
+        indexFiles.add(defaultIndexFile);
+      }
+    }
+
     // Setup service worker schematic options
     const { title, ...swOptions } = options;