fix(css): hoist external @import for non-split css

[sapphi-red]

Description

fixes #5323

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[patak-dev]

Is this fixing a regression in 2.9 or the problem was already present before? The fix looks straightforward enough to back port it to the v2 branch and include it in 2.9.x

[sapphi-red]

This problem was already present at 2.6. I don't know when it started happening.