fix(plugin-legacy): legacy sourcemap not generate (fix #11693)

[sun0day]

Description

fix #11693

Additional context

plugin-legacy should compatible with rollup3.x: don't delete sourcemap in bundle

See rollup3.x break changes

When using the API and sourcemaps, sourcemap comments are contained in the emitted files and sourcemaps are emitted as regular assets (#4605)

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[sapphi-red]

Thank you for the PR!
Would you add a test for this?

[sun0day]

Thank you for the PR! Would you add a test for this?

Done

[sapphi-red]

Thanks!