qmlimportscanner doesn't find non-installed qtdeclarative qml modules in a prefix build

This shows up when trying to build examples as external projects in the CI which uses a prefix build: QTBUG-96232

qtdeclarative is built, but not installed, and the example external projects are using the export(EXPORT) build time Config files.

While the build dir CMake packages themselves are found, _qt_internal_scan_qml_imports passes ${QT6_INSTALL_PREFIX}/${QT6_INSTALL_QML} as an -importPath argument which ends up empty, because qtdeclarative is not installed yet.

It would be nice to make this scenario work, so that the example external projects are actually runnable in a prefix build.
This might also affect in-tree tests, but haven't confirmed yet.

This was discussed a few times internally and also in some jira issues.

We already do something similar (ensuring build time import paths are found in a prefix build) when building Qt itself for qmllint / qmltc via the _qt_internal_extend_qml_import_paths function.