Normalize import paths for Sky modules

sky/tests/framework/xmlhttprequest/xhr-does-not-exist.sky

 <sky>
-<import src="/sky/tests/resources/chai.sky" />
-<import src="/sky/tests/resources/mocha.sky" />
+<import src="../../resources/chai.sky" />
+<import src="../../resources/mocha.sky" />
 <import src="/sky/framework/xmlhttprequest.sky" as="XMLHttpRequest" />
 <script>
 describe("xmlhttprequest", function() {
   it("should call onerror when endpoint does not exist", function(done) {
     var xhr = new XMLHttpRequest();
     xhr.open("GET", "does_not_exist.html");
     xhr.onerror = function() {
       assert.fail("onload", "onerror", "onerror should not be called.");
       done();
     }
     xhr.onload = function() {
       // Missing files are application-level errors, not network errors
       // so onload fires, not onerror.
       assert.equal(xhr.status, 404);
       assert.equal(xhr.statusText, "HTTP/1.1 404 Not Found",
           "status text should also be 404");
       done();
     }
     xhr.send();
   });
 });
 </script>
 </sky>