memory: fix 2 memory leaks in `next-dev` (#43859)
This PR fixes two memory leaks I found debugging with @sokra.
## 1) Leak in `next-server.ts`
The first leak was caused by the fact that the `require.cache` associated to the `next-server` module was not being cleared up properly, so we leaked context from modules required in that page, like API routes.
## 2) Leak with React Fetch
When evaluating a route, we also evaluated the `react.shared-subset.development.js` module where React patches the `fetch` function. The problem is that when re-evaluating a route as part of hot reloading, we were patching over the previously patched `fetch` function.
The result of this operation meant that we were keeping a reference to the context of the previous `fetch` and thus to the previous route context, thus creating a memory leak, since we only needed the new context.
## Test plan
Checked manually the heap snapshots of a test app.
## 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)
