fix(builders): externalize synthesized imports in exposed and mapping bundles

[Aukevanoost]

The shared-mappings plugin from #128 was only wired into the app build. An exposed module that reaches a mapped lib through an NgModule still got Angular's synthesized deep import inlined, shipping a second instance of the lib and its InjectionTokens (NG0201).

• Core 4.7: exposes and mappings build in separate contexts, so the lib is inlined into the expose outright.
• Core 4.6: one context, so it collapses onto the remote's own chunk, but the host's copy of the mapping wins the import map at runtime, so it is still a second instance.

This adds createSharedMappingsPlugin(mappedPaths) to the exposed/mapping build, as suggested in the issue.

Verification

In angular-examples/angular/tractor-store, a probe NgModule (a component that injects an InjectionToken) was added to @ng-internal/ui and used via imports: [ProbeModule] in explore's exposed HomePage:

• Before: the token was in both mfe-home-*.js and _ng_internal_ui-*.js.
• After: the token is only in the mapping bundle, mfe-home imports @ng-internal/ui for both references, and the mapping bundle's hash is unchanged.

Closes #145