Relative include/import schemaLocations should not be resolved absolutely

If a schemaLocation path is relative, then it should only be found relative to the importing schema. Only absolute schemaLocation paths search the classpath. Currently, if a schemaLocation path does not start with a /, it will append one and look on the classpath. This makes it ambiguous if a path should be looked up relative to the importing file or if it should be found on the classpath. This can cause extra complexity down the line.

Note that we may want to fallback to this behavior if a include/import is not found relatively, and output a warning, since many schemas do expect this behavior.