[improvement] rename packages dir to src (#3659)

src/mixins/slots.js

@@ -0,0 +1,16 @@
+/**
+ * Use scopedSlots in Vue 2.6+
+ * downgrade to slots in lower version
+ */
+
+export const SlotsMixin = {
+  methods: {
+    slots(name = 'default', props) {
+      const { $slots, $scopedSlots } = this;
+      if ($scopedSlots[name]) {
+        return $scopedSlots[name](props);
+      }
+      return $slots[name];
+    }
+  }
+};