`requirement_policy/3` is generated and may not be in Spack's inputs to Clingo.
Currently this is causing warnings like:
```
$ spack spec zlib
/global/u2/t/tgamblin/src/spack/lib/spack/spack/solver/concretize.lp:510:3-43: info: atom does not occur in any rule head:
requirement_policy(Package,X,"one_of")
/global/u2/t/tgamblin/src/spack/lib/spack/spack/solver/concretize.lp:517:3-43: info: atom does not occur in any rule head:
requirement_policy(Package,X,"one_of")
/global/u2/t/tgamblin/src/spack/lib/spack/spack/solver/concretize.lp:523:3-43: info: atom does not occur in any rule head:
requirement_policy(Package,X,"any_of")
/global/u2/t/tgamblin/src/spack/lib/spack/spack/solver/concretize.lp:534:3-43: info: atom does not occur in any rule head:
requirement_policy(Package,X,"any_of")
Input spec
--------------------------------
zlib
Concretized
--------------------------------
zlib@1.2.11%gcc@7.5.0+optimize+pic+shared arch=cray-sles15-haswell
```
- [x] Silence warning with `#defined requirement_policy/3`
7e1890772c