fix(packagesettings): reject cross-level (package level and variant level) settings conflicts for source

[smoparth]

Reject conflicting new resolver "source" and legacy settings during package settings loading, including explicit pre_built: false. Cover both configuration directions with focused tests and document the rule.

Closes: #1368

Pull Request Description

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• PR follows CONTRIBUTING.md guidelines

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[rd4398]

This looks good, I left a non-blocking comment which can be a follow up.

[rd4398]

Thanks for the quick fix!

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