Support named exports with nodejs ESM

[robrichard]

It is currently not possible to use named exports with Relay in a Node.js ESM file. For example, in a brand new project with the latest version of relay-runtime as a dependency, the following code will error:

index.mjs:

import { fetchQuery } from 'relay-runtime'

file:///index.mjs:1
import { fetchQuery } from 'relay-runtime';
         ^^^^^^^^^^
SyntaxError: Named export 'fetchQuery' not found. The requested module 'relay-runtime' is a CommonJS module, which may not support all module.exports as named exports.
CommonJS modules can always be imported via the default export, for example using:

import pkg from 'relay-runtime';
const { fetchQuery } = pkg;

    at #asyncInstantiate (node:internal/modules/esm/module_job:302:21)
    at async ModuleJob.run (node:internal/modules/esm/module_job:405:5)
    at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:660:26)
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:101:5)

When an ESM file imports a common js file (like relay-runtime) Node.js will use cjs-module-lexer to try to determine compatible named exports.

From the node.js docs:

Named exports detection covers many common export patterns, reexport patterns and build tool and transpiler outputs. See cjs-module-lexer for the exact semantics implemented.

In the entry points of react-relay and relay-runtime, the pattern of exporting a property on another object causes cjs-module-lexer to not detect any of named exports.

Example:

// This doesn't work
module.exports = {
  fetchQuery: RelayRuntime.fetchQuery
};

// This does work
const { fetchQuery } = RelayRuntime;
module.exports = {
  fetchQuery
};

In this PR, I updated the entrypoints of react-relay and relay-runtime to allow Node.js's cjs named export detection to work properly. I added a test case to the Gulp build process since to test this we need the final transpiled code, but I am open to other approaches.