fix(plugin-legacy): respect entryFileNames for polyfill chunks for v2.x.x Vite version. Issue #4221

[lisunovdv]

Description

It solves the same as #8247 for v2.x.x

Additional context

close #4221
refs #8180

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[lisunovdv]

@patak-dev please, review

[sapphi-red]

The tests are failing and config.build.entryFileNames does not exist.
I feel this change is a bit risky to backport unless the polyfill chunk names preserve as-is when entryFileNames is not set.

[sapphi-red]

Closing as we're not going to backport this feature. But thanks for the PR.