fix(compiler): look for flat module resources using declaration module path (#15367)

`ngc` would look for flat module resources relative to the flat module index.
`ngc` now looks for flat module resources relative to the `.d.ts` file that declarates
the component.

Fixes #15221

PR Close #15367

packages/compiler-cli/integrationtest/flat_module/public-api.ts

@@ -0,0 +1,10 @@
+/**
+ * @license
+ * Copyright Google Inc. All Rights Reserved.
+ *
+ * Use of this source code is governed by an MIT-style license that can be
+ * found in the LICENSE file at https://angular.io/license
+ */
+
+export * from './src/flat.component';
+export * from './src/flat.module';