UseSWIG: Revert "use $<SOURCE_PROPERTY> genex"

Revert commit 25798c15 (UseSWIG: use $<SOURCE_PROPERTY> genex,
2026-06-05).

Moving source file properties evaluation from configuration
to generation time prevents the possibility to have the same source file,
in one directory, in different targets with different settings.

Revert to the previous implementation not using $<SOURCE_PROPERTY>
generator expression.

Fixes: #28193