fix(gatsby): add main field pointing to transpiled commonjs entry (#9123)

* fix: transpile gatsby to commonjs for better testability

* chore: fix up main entry

* fix: ignore tests too (thanks @pieh)

Author: DSchau

packages/gatsby/.gitignore

@@ -28,3 +28,6 @@ node_modules
 
 decls
 dist
+
+# built files
+cache-dir/commonjs/

packages/gatsby/package.json

@@ -160,7 +160,8 @@
     "website"
   ],
   "license": "MIT",
-  "main": "cache-dir/gatsby-browser-entry.js",
+  "main": "cache-dir/commonjs/gatsby-browser-entry.js",
+  "module": "cache-dir/gatsby-browser-entry.js",
   "peerDependencies": {
     "react": "^16.4.2",
     "react-dom": "^16.4.2"
@@ -173,9 +174,10 @@
     "graphql": "^0.13.2"
   },
   "scripts": {
-    "build": "rimraf dist && npm run build:src && npm run build:internal-plugins && npm run build:rawfiles",
+    "build": "rimraf dist && npm run build:src && npm run build:internal-plugins && npm run build:rawfiles && npm run build:cjs",
     "build:internal-plugins": "copyfiles -u 1 src/internal-plugins/**/package.json dist",
     "build:rawfiles": "copyfiles -u 1 src/internal-plugins/**/raw_* dist",
+    "build:cjs": "babel cache-dir --out-dir cache-dir/commonjs --ignore **/__tests__",
     "build:src": "babel src --out-dir dist --source-maps --ignore **/gatsby-cli.js,**/raw_*,**/__tests__",
     "clean-test-bundles": "find test/ -type f -name bundle.js* -exec rm -rf {} +",
     "prepare": "cross-env NODE_ENV=production npm run build",