To be reported with [1] imports require source. It was missing for
implicit script imports.
`FirScriptConfiguratorExtensionImpl` was extended and now adds fake
source elements (users don't see imports in the source code).
Since diagnostics for implicit import statements are meaningless for
IDE, they are suppressed in `LLFirDiagnosticReporter`.
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[1]: DiagnosticReporter.reportOn()