[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...

Reproduce the PWA build using the baseHref values in angular.json and inspect the generated ngsw.json. Start with the build path that constructs service-worker URLs; done means a file-valued baseHref keeps index.html as the index while asset URLs use its containing path, without the duplicated /index.html/ segment.

Written by the indexing model from the issue text.