fix(plugin-legacy): Restore guessable polyfill id (fix #6097)

[StefSchenkelaars]

Description

The name of the polyfill file is determined by a hardcoded string.
Some packages based on this plugin rely on this predefined name.
This name was changed in 3127219
to prevent resolving files which don't exist. This commit changes
that behaviour to restore the predefined name.

Closes #6097

Additional context

I wanted to write a proper test for this but I wasn't sure where to put this. So if anyone can give me some guidelines I could spend some time on that.

Next to this, we are a bit doubting if this was a typo or an intended change as discussed in ElMassimo/vite_ruby#156 and nystudio107/craft-vite#20.

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.
• Read the Pull Request Guidelines and follow the Commit Convention.
• 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).
• Ideally, include relevant tests that fail without this PR but pass with it.

[StefSchenkelaars]

Closed in favor of #6225