Replace prepack and postpac scripts with custom Yarn plugin

As suggested in yarnpkg/berry#3609 (comment)

.yarn/plugins/plugin-remove-workspaces-field.cjs

@@ -0,0 +1,10 @@
+module.exports = {
+  name: `plugin-remove-workspaces-field`,
+  factory: () => ({
+    hooks: {
+      beforeWorkspacePacking(workspace, rawManifest) {
+        delete rawManifest.workspaces;
+      },
+    },
+  }),
+};

.yarnrc.yml

@@ -1,6 +1,7 @@
 nodeLinker: node-modules
 
 plugins:
+  - path: .yarn/plugins/plugin-remove-workspaces-field.cjs
   - path: .yarn/plugins/@yarnpkg/plugin-interactive-tools.cjs
     spec: "@yarnpkg/plugin-interactive-tools"
 

package.json

@@ -11,10 +11,7 @@
     "build": "babel --source-maps=both src -d build",
     "clean": "rimraf build",
     "lint": "eslint src/ --ext .jsx,.js",
-    "postpack": "yarn restore-workspaces",
-    "prepack": "yarn clean && yarn build && yarn remove-workspaces",
-    "remove-workspaces": "node remove_workspaces.js",
-    "restore-workspaces": "node restore_workspaces.js",
+    "prepack": "yarn clean && yarn build",
     "test": "yarn lint"
   },
   "keywords": [