refactor(@angular-devkit/build-angular): move dev-server builder lo…

…gic out of index file

The `index.ts` file for the `dev-server` builder was becoming quite large and it was not
obvious what was exported for external usage and what was exported to support testing.
This change also allows for future extension of the development server without bringing in
all of the Webpack-specific functionality.

goldens/public-api/angular_devkit/build_angular/index.md

@@ -105,8 +105,29 @@ export enum CrossOrigin {
     UseCredentials = "use-credentials"
 }
 
-// @public (undocumented)
-export type DevServerBuilderOptions = Schema;
+// @public
+export interface DevServerBuilderOptions {
+    allowedHosts?: string[];
+    browserTarget: string;
+    disableHostCheck?: boolean;
+    headers?: {
+        [key: string]: string;
+    };
+    hmr?: boolean;
+    host?: string;
+    liveReload?: boolean;
+    open?: boolean;
+    poll?: number;
+    port?: number;
+    proxyConfig?: string;
+    publicHost?: string;
+    servePath?: string;
+    ssl?: boolean;
+    sslCert?: string;
+    sslKey?: string;
+    verbose?: boolean;
+    watch?: boolean;
+}
 
 // @public
 export type DevServerBuilderOutput = DevServerBuildOutput & {
@@ -122,7 +143,7 @@ export function executeBrowserBuilder(options: BrowserBuilderOptions, context: B
 }): Observable<BrowserBuilderOutput>;
 
 // @public
-export function executeDevServerBuilder(options: DevServerBuilderOptions, context: BuilderContext, transforms?: {
+export function executeDevServerBuilder(options: DevServerBuilderOptions_2, context: BuilderContext, transforms?: {
     webpackConfiguration?: ExecutionTransformer<webpack.Configuration>;
     logging?: WebpackLoggingCallback;
     indexHtml?: IndexHtmlTransform;
@@ -154,7 +175,7 @@ export function executeServerBuilder(options: ServerBuilderOptions, context: Bui
 export type ExecutionTransformer<T> = (input: T) => T | Promise<T>;
 
 // @public (undocumented)
-export type ExtractI18nBuilderOptions = Schema_2;
+export type ExtractI18nBuilderOptions = Schema;
 
 // @public (undocumented)
 export interface FileReplacement {