fix: reset dupe plugin tracker on dev server close (#486)

Fixes false positive warnings like this in some cases:

```
        Multiple instances of @netlify/vite-plugin have been loaded. This may cause unexpected
        behavior if the plugin is configured differently in each instance. If you have one
        instance of this plugin in your Vite config, you may safely remove it.
```

Author: serhalp

packages/vite-plugin/src/main.test.ts

@@ -98,6 +98,25 @@ describe.for([['5.0.0'], ['6.0.0'], ['7.0.0']])('Vite %s', ([viteVersion]) => {
       await server.close()
     })
 
+    test('does not warn about duplicate plugin instances when server restarts', async () => {
+      const mockLogger = createMockViteLogger()
+
+      const firstServer = await startTestServer({
+        customLogger: mockLogger,
+        plugins: [netlify({ middleware: false })],
+      })
+      expect(mockLogger.warn).not.toHaveBeenCalled()
+      await firstServer.server.close()
+
+      const secondServer = await startTestServer({
+        customLogger: mockLogger,
+        plugins: [netlify({ middleware: false })],
+      })
+      expect(mockLogger.warn).not.toHaveBeenCalled()
+
+      await secondServer.server.close()
+    })
+
     test('Populates Netlify runtime environment (globals and env vars)', async () => {
       const fixture = new Fixture()
         .withFile(

packages/vite-plugin/src/main.ts

@@ -91,6 +91,11 @@ export default function netlify(options: NetlifyPluginOptions = {}): any {
       await netlifyDev.start()
 
       viteDevServer.httpServer.once('close', () => {
+        // The whole vite dev instance has stopped, so reset the duplicate plugin tracker.
+        // For example, this happens when a user makes a change to `vite.config.ts` and a new server starts.
+        // We shouldn't print a false positive warning in these cases.
+        delete process.env.__VITE_PLUGIN_NETLIFY_LOADED__
+
         netlifyDev.stop()
       })