Add module as server fallback main field (#6256)

* Add `module` as server fallback main field

* Test that a module only package can be imported

packages/next/build/webpack-config.js

@@ -189,7 +189,7 @@ export default async function getBaseWebpackConfig (dir, {dev = false, isServer
       [PAGES_DIR_ALIAS]: path.join(dir, 'pages'),
       [DOT_NEXT_ALIAS]: distDir
     },
-    mainFields: isServer ? ['main'] : ['browser', 'module', 'main']
+    mainFields: isServer ? ['main', 'module'] : ['browser', 'module', 'main']
   }
 
   const webpackMode = dev ? 'development' : 'production'

test/integration/config/node_modules/module-only-package/index.js

@@ -0,0 +1 @@
+module.exports = "I am sometimes found by tooling. I shouldn't be."

test/integration/config/node_modules/module-only-package/modern.js

@@ -0,0 +1 @@
+export default 'OK'

test/integration/config/node_modules/module-only-package/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "module-only-package",
+  "description": "I'm a hipster package that only ships a module entrypoint.",
+  "module": "./modern.js"
+}

test/integration/config/pages/module-only-content.js

@@ -0,0 +1,3 @@
+import messageInAPackage from 'module-only-package'
+
+export default () => <p id='messageInAPackage'>{messageInAPackage}</p>

test/integration/config/test/index.test.js

@@ -35,7 +35,8 @@ describe('Configuration', () => {
     await Promise.all([
       renderViaHTTP(context.appPort, '/next-config'),
       renderViaHTTP(context.appPort, '/build-id'),
-      renderViaHTTP(context.appPort, '/webpack-css')
+      renderViaHTTP(context.appPort, '/webpack-css'),
+      renderViaHTTP(context.appPort, '/module-only-component')
     ])
   })
   afterAll(() => {

test/integration/config/test/rendering.js

@@ -33,5 +33,10 @@ export default function ({ app }, suiteName, render, fetch) {
       const $ = await get$('/build-id')
       expect($('#buildId').text() === '-')
     })
+
+    test('correctly imports a package that defines `module` but no `main` in package.json', async () => {
+      const $ = await get$('/module-only-content')
+      expect($('#messageInAPackage').text() === 'OK')
+    })
   })
 }