Switch to yarnpkg

babel can no longer be built by npm as they use yarn specific features in
package.json. So use yarnpkg everywhere.

Gbp-Pq: Name switch-to-yarnpkg.patch

babel-plugin-dynamic-import-node/package.json

@@ -8,7 +8,7 @@
   },
   "scripts": {
     "clean": "rimraf lib",
-    "prebuild": "npm run clean",
+    "prebuild": "yarnpkg clean",
     "build": "babel src --out-dir lib",
     "pretest": "npm run lint",
     "test": "npm run tests-only",