[fix] babel-plugin only rewrites paths for known modules

Don't rewrite import paths for non-existent modules or types. They will
attempt to be imported from the package's main export. This change
currently requires a module map to be generated for the babel-plugin to
use. The map is automatically regenerated for any commit that alters the
entry file of react-native-web.

Fix #822

package.json

@@ -62,6 +62,9 @@
       "fmt:cmd",
       "git update-index --again",
       "eslint"
+    ],
+    "packages/react-native-web/src/index.js": [
+      "node ./scripts/babel/createModuleMap.js"
     ]
   },
   "prettier": {