title: Angular CLI 9 (Ivy) App build with local library (mono-repo) failing
body: <p>I have an Angular 9 app that i'm starting to move some of the components into a dedicated library project, which uses secondary entry points (trying to mimic Angular Material's approach).  Everything builds fine if i build both library and app with the Ivy compiler, but if build both with the --prod flag (which causes the library to be built without Ivy), the app build fails with the following error:</p>
  
  <pre><code>Compiling @fabric/components/breadcrumbs : es2015 as esm2015
  
  ERROR in node_modules/@fabric/components/breadcrumbs/fab-breadcrumbs.module.d.ts:1:22 - error NG6002: Appears in the NgModule.imports of AppModule, but could not be resolved to an NgModule class.
  
  This likely means that the library (@fabric/components/breadcrumbs) which declares FabBreadcrumbsModule has not been processed correctly by ngcc, or is not compatible with Angular Ivy. Check if a newer version of the library is available, and update if s
  o. Also consider checking with the library's authors to see if the library is expected to be compatible with Ivy.
  
  1 export declare class FabBreadcrumbsModule {
                         ~~~~~~~~~~~~~~~~~~~~
  </code></pre>
  
  <p>I tried updating to the latest version (9.0.2 -> 9.1.0), but it's still doing the same thing.  I tried recreating this with a brand new super simple mono-repo project, but in that simple case it works fine.  I compared the tsconfig and angular.json config files between my app and the simple working app, but they're fairly identical. So i'm kind of stuck.  I'm not sure what would be causing this, whether it's on the ng-packagr side, not building the library correctly, or if it's on the angular-build on the app side that's messed up.  Any advice would be greatly appreciated!</p>