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

src/dropdown-item/index.less

@@ -0,0 +1,25 @@
+@import '../style/var';
+
+.van-dropdown-item {
+  position: fixed;
+  right: 0;
+  left: 0;
+  overflow: hidden;
+
+  &--up {
+    top: 0;
+  }
+
+  &--down {
+    bottom: 0;
+  }
+
+  &__content {
+    position: absolute;
+  }
+
+  &__icon {
+    display: block;
+    line-height: inherit;
+  }
+}