fix(server): only warn sourcemap in `loadTransform` when explicitly turned on

[antfu]

Description

We got quite some warnings regarding sourcemap when we never need them, and it seems there is no way to disable it:

This kind of warning is generally useless but quite annoying to the end users. I think it make more sense only to warn when sourcemap is enabled.

Additional context

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What is the purpose of this pull request?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

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

• [x] Read the Contributing Guidelines, especially the Pull Request Guidelines.
• [x] 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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[bluwy]

@antfu are you still interested in pushing forward this PR? I had update the warning in https://github.com/vitejs/vite/pull/16170 to print the entire error instead, and I usually find actual configuration error that should be fixed instead of hiding.

[bluwy]

I'll close this for now as the updated warning should be more helpful now.