fix: make addWatchFile() work (fix #7024)

[zqianem]

Description

fixes #7024 by ensuring addWatchFile adds files to the module graph

Additional context

The intention of db4ba56 was to allow plugins to add files to the module graph via addWatchFile() so that changes to those files would cause HMR updates. However, for files without any ES imports, the import analysis returns early and the files passed to addWatchFile() in this._addedImports are never added to the graph. Checking for the existence of this._addedImports fixes this.

Additionally, the new forceSkipImportAnalysis flag is needed to mark the newly created modules as not needing additional analysis so that the HMR update can propogate (see #8898).

I could try to add the tests from #4461 if they are still relevant. Cherry-picked the test from #4461 and verified that it fails without 33938a3.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[zqianem]

Rebased onto latest main and re-ordered the commits (add tests first) for easier verification of fix.

[zqianem]

Rebased again to verify that the fix is still necessary (the added test fails without) and that it still works. I think the failed tests are due to flakiness — different tests fail on the same commit in my fork.

@patak-dev could you please take a look at this? I'm worried that this PR has missed the triage process due to lack of labels and wasn't sure how to reach out; apologies if this is already on the radar. Thanks!

[patak-dev]

@zqianem thanks for the ping (and for the patience). Looks good to me 👍🏼
I will send it to #contributing to https://chat.vitejs.dev to get another review or merge it tomorrow if not because we can test it safely during the beta.