perf(transformRequest): fast-path watch and sourcemap handling

[bluwy]

Description

1. Skip ensureWatchedFile if the file failed to load from the fs.
2. Skip removing the sourcemap comment from files if we found there's no sourcemap early on.
3. Improves sourcemap error message. (Users were having trouble with it in the past: Failed to load source map for /node_modules/.vite/vite.js #6981 and can't read sourcemap file #10172)

Additional context

I found these while trying to make this.load also work for files in the filesystem (#16018 (comment)), I needed to share some code from transformRequest.ts to pluginContainer.ts. But this PR applies to refactors first so it's easier to review.

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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, especially the Pull Request Guidelines.
• 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).
• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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[sapphi-red]

I found these while trying to make this.load also work for files in the filesystem (#16018 (comment)), I needed to share some code from transformRequest.ts to pluginContainer.ts.

I thought about that while creating #14953, too. 🙂

[patak-dev]

/ecosystem-ci run

[vite-ecosystem-ci]

📝 Ran ecosystem CI on a04d7a5: Open

suite	result	latest scheduled
vite-plugin-react-pages	✅ success	❌ failure

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