[BUG][PWA] Url error in the "ngsw.json" file with a "baseHref" pointing to a file (eg: "/index.html")

### Bug Report or Feature Request
```
- [X] bug report -> please search issues before submitting
- [ ] feature request
```
From here: https://github.com/angular/angular/issues/26917

### Command
```
- [X] build
```

### Versions
```
Angular CLI: 7.0.4
Node: 10.8.0 (npm: 6.3.0)
OS: win32 x64
Angular: 7.0.2
... animations, cdk, common, compiler, compiler-cli, core, forms
... http, language-service, material, platform-browser
... platform-browser-dynamic, router, service-worker

Package Version
-----------------------------------------------------------
@angular-devkit/architect 0.10.4
@angular-devkit/build-angular 0.10.4
@angular-devkit/build-optimizer 0.10.4
@angular-devkit/build-webpack 0.10.4
@angular-devkit/core 7.0.4
@angular-devkit/schematics 7.0.4
@angular/cli 7.0.4
@angular/pwa 0.10.4
@ngtools/webpack 7.0.4
@schematics/angular 7.0.4
@schematics/update 0.10.4
rxjs 6.3.3
typescript 3.1.6
webpack 4.19.1
```

## Current behavior
I have a problem with the file `ngsw.json` (`dist/xxx/ngsw.json`), when I build my application.
Here is the configuration of my file `angular.json`:
```json
{
"$schema": "./node_modules/@angular/cli/lib/config/schema.json",
"version": 1,
"newProjectRoot": "projects",
"projects": {
"xxxxxx-xxxxxx-web": {
...
"architect": {
"build": {
"builder": "@angular-devkit/build-angular:browser",
"options": {
"baseHref": "/index.html", <-- baseHref with path to a file
...
},
"configurations": {
"develop": {
...
"baseHref": "/dev-web-xxxxxx-xxxxxx/index.html", <-- baseHref with path to a file
"fileReplacements": [
{
"replace": "src/environments/environment.ts",
"with": "src/environments/environment.develop.ts"
}
],
"optimization": true,
"outputHashing": "all",
"sourceMap": true,
"extractCss": true,
"namedChunks": false,
"aot": true,
"extractLicenses": true,
"vendorChunk": false,
"buildOptimizer": true,
"serviceWorker": true
}
...
```
As you can see, my `baseHref` points to a file (`index.html`). And when I compile, here is the result of the file `ngsw.json`:
```json
{
"configVersion": 1,
"index": "/dev-web-xxxxxx-xxxxxx/index.html/index.html",
"assetGroups": [ ^^^^^^^^^^
{
"name": "app",
"installMode": "prefetch",
"updateMode": "prefetch",
"urls": [ vvvvvvvvvv
"/dev-web-xxxxxx-xxxxxx/index.html/4.459d2c031076fa54578c.js",
"/dev-web-xxxxxx-xxxxxx/index.html/5.b0a92526f07730e41004.js",
"/dev-web-xxxxxx-xxxxxx/index.html/index.html",
"/dev-web-xxxxxx-xxxxxx/index.html/main.284d15247a9766d625e1.js",
"/dev-web-xxxxxx-xxxxxx/index.html/polyfills.f8bea440aac573c30541.js",
"/dev-web-xxxxxx-xxxxxx/index.html/runtime.e04cbf91ceee4f9b3494.js",
"/dev-web-xxxxxx-xxxxxx/index.html/styles.380a38f67b77a997829f.css"
], ^^^^^^^^^^
"patterns": []
},
{
"name": "assets",
"installMode": "lazy",
"updateMode": "prefetch",
"urls": [ vvvvvvvvvv
"/dev-web-xxxxxx-xxxxxx/index.html/assets/icons/icon-128x128.png",
"/dev-web-xxxxxx-xxxxxx/index.html/assets/icons/icon-144x144.png",
... ^^^^^^^^^^
```
As you can see, it generates all the urls of the file with the `.../index.html/...`, as if it were a folder. As this file is used for the cache of the application, the cache does not work since it is not the right url...

## Expected behavior

I think the file should have looked like this:
```json
{
"configVersion": 1,
"index": "/dev-web-xxxxxx-xxxxxx/index.html",
"assetGroups": [
{
"name": "app",
"installMode": "prefetch",
"updateMode": "prefetch",
"urls": [
"/dev-web-xxxxxx-xxxxxx/4.459d2c031076fa54578c.js",
"/dev-web-xxxxxx-xxxxxx/5.b0a92526f07730e41004.js",
"/dev-web-xxxxxx-xxxxxx/index.html",
"/dev-web-xxxxxx-xxxxxx/main.284d15247a9766d625e1.js",
"/dev-web-xxxxxx-xxxxxx/polyfills.f8bea440aac573c30541.js",
"/dev-web-xxxxxx-xxxxxx/runtime.e04cbf91ceee4f9b3494.js",
"/dev-web-xxxxxx-xxxxxx/styles.380a38f67b77a997829f.css"
],
"patterns": []
},
{
"name": "assets",
"installMode": "lazy",
"updateMode": "prefetch",
"urls": [
"/dev-web-xxxxxx-xxxxxx/assets/icons/icon-128x128.png",
"/dev-web-xxxxxx-xxxxxx/assets/icons/icon-144x144.png",
...
```

## Minimal reproduction of the problem with instructions

You just have to create an application (which uses PWA) and have a baseHref that points to a file (eg: `index.html`). And when you compile your application, the url generated in the `ngsw.json` file will be wrong.

## What is the motivation / use case for changing the behavior?

I think it would just take into account when the "baseHref" to deal with a file, take the root of the file (and not take the file for a folder).

For information, I use a `baseHref` to a file, because I use the system of route of Angular with the hash and that the url `https://storage.googleapis.com/dev-web-xxxxxx-xxxxxx/` does not point towards my file `index.html`. As the project is hosted on a Google Cloud Storage, when we update the application (CTRL+F5, ...) with a url of this type `...-xxxxxx/#/user/login`, I end up with a 404 error (because the url in `...-xxxxxx/` does not point to the file `index.html`). To solve this problem, I force Angular to keep the `index.html` in the URL (eg: `...-xxxxxx/index.html#/user/login`), through `baseHref` with the `index.html` in it.

For now, I make the corrections of the url in the file `ngsw.json` manually, so that the cache of the application works...

Reproduisez le build PWA en utilisant les valeurs de baseHref dans angular.json et inspectez le ngsw.json généré. Commencez par le chemin de build qui construit les URL du service worker ; c’est terminé lorsqu’un baseHref correspondant à un fichier conserve index.html comme index, tandis que les URL des assets utilisent le chemin contenant ce fichier, sans le segment dupliqué /index.html/.

Rédigé par le modèle d'indexation à partir du texte de l'issue.