fix(importGlob): don't warn when CSS default import is not used

[sapphi-red]

Description

I implemented #11103 (comment) by moving the warning to runtime.
Since we only output the warning only on dev, I think we could move it.

This PR will change a property to getter. Though, I think this change won't break things most of the time.

refs #11094 #11103

Additional context

Note that sourcemap is currently broken. See #11122.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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[bluwy]

Neat trick. Only one comment for cleanup but this is much nicer DX-wise.