linter: fix match namespace handling

closes #601

scripts/lint.py

@@ -511,7 +511,7 @@ def get_normpath(path):
 def get_features(ctx, rule):
     # get features from rule and all dependencies including subscopes and matched rules
     features = []
-    namespaces = capa.rules.index_rules_by_namespace([rule])
+    namespaces = ctx["rules"].rules_by_namespace
     deps = [ctx["rules"].rules[dep] for dep in rule.get_dependencies(namespaces)]
     for r in [rule] + deps:
         features.extend(get_rule_features(r))