fix(ssr): skip dedupe require in esm

[bluwy]

Description

The dedupeRequire function injected in the build only works in CJS, so we exclude it if we're building for ESM.

Additional context

Caused by #3951. (cc @aleclarson) I've confirmed this fixes the build tests in vite-plugin-svelte.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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