qmake: Correctly handle shadow build of nested vcsubdirs projects.

For some reason, the solution generator was looking for the vcproj
files in the source tree. It should look for them in the output tree
instead (suggested by Joerg Bornemann). This should handle both
in-source and out-of-source builds, and the special-case code for
handling out-of-source builds (which had a bug) can be removed.

Task-number: QTBUG-49665
Change-Id: I40b5c5907c52ffb074ccb8f297bb5924eacc1cb0
Reviewed-by: Oliver Wolff <oliver.wolff@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@theqtcompany.com>