check itemgroup to exist and be instance of ModifiableTopLevelItemGroup
fix name .substring

Not so visible an effect currently due to a bug fixed in 4ace69e (maven-hpi-plugin 1.87)
by which extensions of all plugins which are test deps of jenkins-test-harness were being loaded inadvertently.
Originally tried to define ${bundledPlugins} using gmaven-plugin as per [1] but this does not work,
so adopted the tack of just using a static glob *.hpi and interpreting that in core.
[1] http://stackoverflow.com/questions/7601835/why-cant-the-maven-deploy-plugin-resolve-my-custom-system-property

fix JENKINS-14772

as get(String idOrFullName) suggest

so that getAll() don't invoke CanonicalIdResolvers for known IDs