build: add missing path mapping for build-webpack import causing lint error

I suspect there were some versioning changes with the e.g. `hoist =
false` setting in npmrc; so eslint now starts failing about an import
from `webpack-server.ts` resulting in unnecessary type cast lint errors.

The existing mapping didn't work due to the underscore conversion, so
this makes sense, and fixing the path mappings works.

Author: devversion

packages/angular_devkit/build_angular/src/builders/dev-server/webpack-server.ts

@@ -263,7 +263,7 @@ export function serveWebpackBrowser(
             ...buildEvent,
             baseUrl: serverAddress,
             stats: generateBuildEventStats(webpackRawStats, browserOptions),
-          };
+          } as DevServerBuilderOutput;
         }),
       );
     }),

tsconfig.json

@@ -25,6 +25,7 @@
       "@angular-devkit/schematics/tools": ["./packages/angular_devkit/schematics/tools/index"],
       "@angular-devkit/schematics/testing": ["./packages/angular_devkit/schematics/testing/index"],
       "@angular-devkit/architect/testing": ["./packages/angular_devkit/architect/testing/index"],
+      "@angular-devkit/build-webpack": ["./packages/angular_devkit/build_webpack"],
       "@angular-devkit/*": ["./packages/angular_devkit/*/src"],
       "@angular/*": ["./packages/angular/*/src"],
       "@angular/build/private": ["./packages/angular/build/src/private"],