build: Do not provide built sources as libmutter_dep sources

This is essentially a revert of
https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/326. This commit
had the unintended side effect that the built sources are actually
rebuilt for every individual user of libmutter_dep. With there being more
tests and generated files, the number of targets to build is increasing
squarely.

Not doing this reduces the number of targets from 2044 to 874, thus
saving man hours and CI burnt cycles in the long run. There's the slight
risk of reintroducing the random build breaks, but mutter is essentially
doing as suggested at https://github.com/mesonbuild/meson/issues/1084
(the only difference being addressed in the previous commit), so meson
ought to behave as expected.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1458