Do not drop aggregated changes with resumed compilation

[vladar]

Description

This PR aims to fix an issue with suspend and resume flow of webpack Watching (see #12882). The root cause of the issue is that watchpack instance is closed before aggregated event is emitted (but after the change event). So aggregated changes are silently dropped.

I've added the simplest test to demonstrate this but there are other code paths where the same underlying problem occurs.

The fix I've added so far is just a draft to demonstrate a possible solution. But ideally, we should patch watchpack to allow flushing pending aggregated events when closing the watcher.

Can open a PR in watchpack with required changes. But first, want to make sure it is a viable solution.

What kind of change does this PR introduce?

A bugfix

Did you add tests for your changes?

Yes

Does this PR introduce a breaking change?

An anticipated fix should be backwards compatible.

What needs to be documented once your changes are merged?

This PR doesn't require documentation.

Fixes #12882

[webpack-bot]

For maintainers only:

• This needs to be documented (issue in webpack/webpack.js.org will be filed when merged)
• This needs to be backported to webpack 4 (issue will be created when merged)

[webpack-bot]

Thank you for your pull request! The most important CI builds succeeded, we’ll review the pull request soon.

[tu4mo]

Any chance getting this reviewed soon?

[sokra]

Merged by #13399