This reverts commit 224a60989e.
Turns out that we cannot just untangle the determination of
source root and build root, because this breaks the assumption
that every .qmake.conf results in a separate .qmake.cache in the
build tree. QTBUG-76140 must be fixed differently.
Fixes: QTBUG-76907
Change-Id: I5c0a3719d5e00a0f1cacad51651b47c1f284d22d
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>