use a relative path, not an absolute one!

all uses of Plasma/Generic packages checked so far still work and
package structure tests pass

REVIEW:108664

plasma/packagestructure.cpp

@@ -141,7 +141,7 @@ PackageStructure::Ptr PackageStructure::load(const QString &packageFormat)
     } else if (packageFormat == "Plasma/Generic") {
         structure = defaultPackageStructure(GenericComponent);
         structure->d->type = "Plasma/Generic";
-        structure->setDefaultPackageRoot(KStandardDirs::locate("data", "plasma/packages/"));
+        structure->setDefaultPackageRoot("plasma/packages/");
     }
 
     if (structure) {