IO::getResource is not rooted

[jurgenvinju]

Describe the bug

rascal>getResource("salix/demo/shop/test.css")
|std:///IO.rsc|(7029,2,<129,56>,<129,58>): IO("salix/demo/shop/test.css found more than once: {|mvn://org.rascalmpl!salix-core!0.2.3/src/main/rascal/salix/demo/shop/test.css|,|mvn://org.rascalmpl!salix-core!0.2.3/salix/demo/shop/test.css|}")
	at getResource(|std:///IO.rsc|(5881,1158,<103,0>,<131,1>))
	at $shell$(|prompt:///|(0,40,<1,0>,<1,40>)ok

The file occurs twice indeed:

• once at the root
• once nested under src

But getResources is not supposed to go searching through the entire jar, imo. That's wasteful, especially since we always know what the file is we are searching for, just not in which jar file it is located currently.