Swap rsync for cp --archive for module sources copying

Swap rsync for cp --archive for module sources copying

As Tobias reported, rsync is a bit of heavyweight dependency. We
introduced it, as a replacement for the rm/cp -r previously used.

The rsync was inspired since, unlike make, meson will build all the test
binaries/artefacts even without calling "meson test".

We can go back to cp with --archive (--preserve=timestamps at least),
which will ensure we don't get stale files. To ensure the second run
doesn't copy the source folder as _subfolder_ of the dest we need to
wildcard the copy... Plus we need a proper destination folder in the
first place.

With this, we get a no-op second+ builds - be that with meson or make.
Since the explicit always-dirty state is by design, drop the meson TODO
and document the output variable.

Confirmed by comparing both the `make --debug` output and the execution
times.

Reported-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://github.com/kmod-project/kmod/pull/192
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>