use uri:classloader://META-INF/jruby.home for all cases with a jar-context

uri:classloader is the classloader whichi loaded jruby itself.
cleanup GEM_PATH when evaluating it

when populating the Gem::Specification.dirs from GEM_PATH and the found
specifications directories in the classloader, make sure the uri-like pathes
are all understood by jruby and do indeed exists.