checksums: use sha256 checksums everywhere

We'd like to use a consistent checksum scheme everywhere so that we can:

    a) incorporate archive checksums into our specs and have a
       consistent hashing algorithm across all specs.

    b) index mirrors with a consistent type of checksum, and not one that
       is dependent on how spack packages are written.

- [x] convert existing md5, sha224, sha512, sha1 checksums to sha256

var/spack/repos/builtin/packages/clingo/package.py

@@ -19,7 +19,7 @@ class Clingo(CMakePackage):
     homepage = "https://potassco.org/clingo/"
     url      = "https://github.com/potassco/clingo/archive/v5.2.2.tar.gz"
 
-    version('5.2.2', 'd46a1567f772eebad85c6300d55d2cc3')
+    version('5.2.2', sha256='da1ef8142e75c5a6f23c9403b90d4f40b9f862969ba71e2aaee9a257d058bfcf')
 
     depends_on('doxygen', type=('build'))
     depends_on('python')