VS: Fix regression causing source accumulation across Fortran projects

Since commit de2afc5d53 (cmMakeFile: Remove source groups argument from
`FindSourceGroup`, 2025-11-07, v4.3.0-rc1~414^2~2) we share source group
assignments across targets to avoid copying and repeated work.  However,
the VS 7 generator, still used for Fortran, relies on per-target
per-group source lists.  Build them specifically for that case.

The deprecated Eclipse CDT4 extra generator had the same regression,
so fix it too.

Fixes: #27779