Fix mixed exports in server component with barrel optimization (#64894)

### What

* Remove the change added in #64467, but still kept tests.
* Add more tests for mixed imports (component and function) from shared
component with `optimizePackageImports`

### Why

The fix in #64467 was not correct, as mixing `export *` with `"use
client"` should error if Next.js detect it properly.
When there's mixed exports, that a shared function will become a client
reference if the target file inherits the client boundary.

The original issue #64467 fixed was actually related to tree-shaking,
which is fixed in #64681.

Closes NEXT-3197

packages/next/src/build/webpack/loaders/next-barrel-loader.ts

@@ -225,8 +225,6 @@ async function getBarrelMapping(
           if (targetMatches) {
             // Merge the export list
             exportList = exportList.concat(targetMatches.exportList)
-            // Inherit the client boundary from the target matched file
-            isClientEntry = isClientEntry || targetMatches.isClientEntry
           }
         })
       )

test/production/app-dir/barrel-optimization/basic/app/mixed-barrel-imports/page.js

@@ -0,0 +1,9 @@
+import { buttonStyle, Button } from 'mixed-lib'
+
+export default function Page() {
+  return (
+    <div>
+      <Button id="component" style={buttonStyle()} />
+    </div>
+  )
+}

test/production/app-dir/barrel-optimization/basic/index.test.ts

@@ -8,10 +8,14 @@ describe('Skipped in Turbopack', () => {
     },
     ({ next }) => {
       it('should build successfully', async () => {
-        // Ensure that MUI is working
         const $ = await next.render$('/')
         expect(await $('#client-mod').text()).toContain('client:default')
       })
+
+      it('should handle mixed imports from barrel optimized lib correctly', async () => {
+        const $ = await next.render$('/mixed-barrel-imports')
+        expect(await $('#component').attr('style')).toContain('color:blue')
+      })
     }
   )
 })

test/production/app-dir/barrel-optimization/basic/next.config.js

@@ -1,5 +1,5 @@
 module.exports = {
   experimental: {
-    optimizePackageImports: ['my-lib'],
+    optimizePackageImports: ['my-lib', 'mixed-lib'],
   },
 }