Subcomponents don't see bindings from parent in multibindings

[quiro91]

Parent's bindings are not present in the child subcomponent when using multibindings. This is clear in the test ModuleWithSubcomponentsTest, currently excluded. In order to fix it, we could check for existing binding from the parent scope in the build() method of Scope.Builder. I can raise a PR if you are OK with this solution

[JakeWharton]

Yeah I believe this is causing https://github.com/JakeWharton/dagger-reflect/blob/6ab4adefb67c315e49e303c973bab557b213afa7/integration-tests/upstream/build.gradle#L76-L77 and maybe even some of the failures above that as well.