feat(upgrade): allow non-element selectors for downgraded components (#14037)

This affects the dynamic version of `upgrade` and makes it more consistent with
the static version, while removing an artificial limitation.

This commit also refactors the file layout and code, in order to share code wrt
to dowgrading components between the dynamic and static versions.

modules/@angular/upgrade/test/static/integration/downgrade_component_spec.ts

@@ -273,6 +273,30 @@ export function main() {
          });
        }));
 
+    it('should allow attribute selectors for downgraded components', async(() => {
+         @Component({selector: '[itWorks]', template: 'It works'})
+         class WorksComponent {
+         }
+
+         @NgModule({
+           declarations: [WorksComponent],
+           entryComponents: [WorksComponent],
+           imports: [BrowserModule, UpgradeModule]
+         })
+         class Ng2Module {
+           ngDoBootstrap() {}
+         }
+
+         const ng1Module = angular.module('ng1', []).directive(
+             'worksComponent', downgradeComponent({component: WorksComponent}));
+
+         const element = html('<works-component></works-component>');
+
+         bootstrap(platformBrowserDynamic(), Ng2Module, element, ng1Module).then((upgrade) => {
+           expect(multiTrim(document.body.textContent)).toBe('It works');
+         });
+       }));
+
     it('should allow attribute selectors for components in ng2', async(() => {
          @Component({selector: '[itWorks]', template: 'It works'})
          class WorksComponent {