Refactor code (#43828)

Didn't modify any test, just merging HMR related ones together.

## Bug

- [ ] Related issues linked using `fixes #number`
- [ ] Integration tests added
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [ ] [e2e](https://github.com/vercel/next.js/blob/canary/contributing/core/testing.md#writing-tests-for-nextjs) tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [ ] Errors have a helpful link attached, see [`contributing.md`](https://github.com/vercel/next.js/blob/canary/contributing.md)

## Documentation / Examples

- [ ] Make sure the linting passes by running `pnpm build && pnpm lint`
- [ ] The "examples guidelines" are followed from [our contributing doc](https://github.com/vercel/next.js/blob/canary/contributing/examples/adding-examples.md)

packages/next/server/dev/hot-reloader.ts

@@ -766,7 +766,7 @@ export default class HotReloader {
       (
         pageHashMap: Map<string, string>,
         changedItems: Set<string>,
-        serverComponentChangeCallback?: (key: string) => void
+        serverComponentChangedItems?: Set<string>
       ) =>
       (stats: webpack.Compilation) => {
         try {
@@ -849,12 +849,12 @@ export default class HotReloader {
                   }
                   pageHashMap.set(key, curHash)
 
-                  if (serverComponentChangeCallback) {
+                  if (serverComponentChangedItems) {
                     const serverKey = WEBPACK_LAYERS.server + ':' + key
                     const prevServerHash = pageHashMap.get(serverKey)
                     const curServerHash = chunksHashServerLayer.toString()
                     if (prevServerHash && prevServerHash !== curServerHash) {
-                      serverComponentChangeCallback(key)
+                      serverComponentChangedItems.add(key)
                     }
                     pageHashMap.set(serverKey, curServerHash)
                   }
@@ -877,16 +877,18 @@ export default class HotReloader {
     )
     this.multiCompiler.compilers[1].hooks.emit.tap(
       'NextjsHotReloaderForServer',
-      trackPageChanges(prevServerPageHashes, changedServerPages, (key) =>
-        changedServerComponentPages.add(key)
+      trackPageChanges(
+        prevServerPageHashes,
+        changedServerPages,
+        changedServerComponentPages
       )
     )
     this.multiCompiler.compilers[2].hooks.emit.tap(
       'NextjsHotReloaderForServer',
       trackPageChanges(
         prevEdgeServerPageHashes,
         changedEdgeServerPages,
-        (key) => changedServerComponentPages.add(key)
+        changedServerComponentPages
       )
     )