Use .mjs for module code

* Writes .mjs instead of .js for esmodules code.
* Moves .mjs out of ./module into the top level since it no longer conflicts with .js

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"
@@ -28,14 +28,13 @@
     "prettier": "prettier --write 'src/**/*.js'",
     "check": "flow check",
     "check-cover": "for file in {src/*.js,src/**/*.js}; do echo $file; flow coverage $file; done",
-    "build": "npm run build:clean && npm run build:cp && npm run build:npm && npm run build:npm-flow && npm run build:module && npm run build:module-flow && npm run build:package-json",
+    "build": "npm run build:clean && npm run build:cp && npm run build:package-json && npm run build:cjs && npm run build:mjs && npm run build:flow",
     "build:clean": "rm -rf ./dist && mkdir ./dist",
     "build:cp": "cp README.md LICENSE ./dist",
     "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:cjs": "babel src --optional runtime --ignore __tests__ --out-dir dist/",
+    "build:mjs": "BABEL_MODULES=1 babel src --optional runtime --ignore __tests__ --out-dir dist/module/ && for file in $(find dist/module -name '*.js'); do mv \"$file\" `echo \"$file\" | sed 's/dist\\/module/dist/g; s/.js$/.mjs/g'`; done && rm -rf dist/module",
+    "build:flow": "for file in $(find ./src -name '*.js' -not -path '*/__tests__*'); do cp \"$file\" `echo \"$file\" | sed 's/\\/src\\//\\/dist\\//g'`.flow; done",
     "preversion": ". ./resources/checkgit.sh && npm test",
     "prepublish": ". ./resources/prepublish.sh",
     "gitpublish": ". ./resources/gitpublish.sh"