Dependencies in the "if_true" keyword argument do not prevent the
sources from being used; in other words, they work just like dependencies
with "disabler: false".
However, this was broken in commit ab0ffc6a2 ("modules/sourceset: Fix
remaining typing issues", 2022-02-23) which changed logic instead of
just fixing typing issues. This was likely an attempt to avoid using
"dependencies.Dependency" after the "dependencies" field was declared,
but it also broke QEMU.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>