[component_manager] Don't assume /svc/fuchsia.pkg.PackageResolver always exists

This is currently only true in tests of component_manager that run as v1
Components, but will not be true of the component_manager started from
bootsvc.

CF-750 #done

Test: No package resolver case is covered by new no_pkg_resolver_test.
Package resolver present case is covered by existing integration tests
like routing_integration_test.

Change-Id: Ide07ab3d87ee5c31ea8f036c7dbb3c3b59c7fc00