Use .mjs for module code

* Writes .mjs instead of .js for code within ./module
* Add a top level index.mjs file (which simply redirects to ./module)

Fixes #1217

package.json

@@ -3,8 +3,8 @@
   "version": "0.13.0",
   "description": "A Query Language and Runtime which can target any service.",
   "license": "MIT",
-  "main": "index.js",
-  "module": "module/index.js",
+  "main": "index",
+  "module": "index.mjs",
   "homepage": "https://github.com/graphql/graphql-js",
   "bugs": {
     "url": "https://github.com/graphql/graphql-js/issues"
@@ -34,8 +34,8 @@
     "build:package-json": "node ./resources/copy-package-json.js",
     "build:npm": "babel src --optional runtime --ignore __tests__ --out-dir dist/",
     "build:npm-flow": "find ./src -name '*.js' -not -path '*/__tests__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/dist\\//g'`.flow; done",
-    "build:module": "BABEL_MODULES=1 babel src --optional runtime --ignore __tests__ --out-dir dist/module/",
-    "build:module-flow": "find ./src -name '*.js' -not -path '*/__tests__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/dist\\/module\\//g'`.flow; done",
+    "build:module": "BABEL_MODULES=1 babel src --optional runtime --ignore __tests__ --out-dir dist/module/ && for file in $(find dist/module -name '*.js'); do mv \"$file\" \"${file%.js}.mjs\"; done && echo \"export * from './module'; /*:: export type * from './module'; */\" > dist/index.mjs",
+    "build:module-flow": "find ./src -name '*.js' -not -path '*/__tests__*' | while read filepath; do cp $filepath `echo $filepath | sed 's/\\/src\\//\\/dist\\/module\\//g; s/.js$/.mjs.flow/g'`; done",
     "preversion": ". ./resources/checkgit.sh && npm test",
     "prepublish": ". ./resources/prepublish.sh",
     "gitpublish": ". ./resources/gitpublish.sh"