fix(@ngtools/webpack): allow path mapping to resolve in JS files

Previously we filtered our path mapping resolver to only be used on typescript.
The correct behaviour is unclear; tsc does not resolve JS files, only TS. But
there is a lot of value to use path mapping to resolve JavaScript files, and
it replaces the webpack alias configuration option. Because of that value it
was decided to fix this.

Fixes angular#8117.

Author: hansl

packages/@ngtools/webpack/src/paths-plugin.ts

@@ -124,7 +124,7 @@ export class PathsPlugin implements Tapable {
     this._nmf.plugin('before-resolve', (request: NormalModuleFactoryRequest,
                                         callback: Callback<any>) => {
       // Only work on TypeScript issuers.
-      if (!request.contextInfo.issuer || !request.contextInfo.issuer.endsWith('.ts')) {
+      if (!request.contextInfo.issuer || !request.contextInfo.issuer.match(/\.[jt]s$/)) {
         return callback(null, request);
       }