Allow urlhandlers to create urls for jrt protocol without an add-opens

The way we currently look-up and create built-in handlers in the urlhandlers requires the handler package to be open to felix. As this is not the case for the jrt handler by default, the framework needs to be started with an add-opens for the java.base/sun.net.www.protocol.jrt package if jrt urls need to be created.

Fortunately. it should be possible to workaround the issue in Felix.