The new implementation of `sort` backported from upstream adds a
dependency on the `rayon` crate. This commit adds the corresponding
Debian package as a build-dependency.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>

As we want `rust-coreutils` to be a replacement to `coreutils-gplv2`, we
need to ensure this package gets picked up when boostrapping the system,
and that `coreutils-gplv2` won't be installed at the same time.

This is achieved by:
- adding Conflicts/Breaks/Replaces/Provides relationships identical to
  those of `coreutils-gplv2`
- adding another Conflicts relationship with `coreutils-gplv2`
- make the package priority `required` so it can be pulled by
  `debootstrap`

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>