VS: set ScanSourceForModuleDependencies at vcxproj level

Implement the target-wide `CXX_SCAN_FOR_MODULES`/`CMP0155` selection
with the `.vcxproj`-wide `ScanSourceForModuleDependencies` setting.
Set the per-source equivalent only when needed for a per-source
`CXX_SCAN_FOR_MODULES` property.

This approach enables Intellisense for interfaces imported from modules.
It is also more consistent with what a user might expect when
investigating the state of module scanning from the VS property panels.

Fixes: #25806
Fixes: #25947