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/cloc/package.py

@@ -14,7 +14,7 @@ class Cloc(Package):
     url      = "https://github.com/AlDanial/cloc/releases/download/1.74/cloc-1.74.tar.gz"
 
     version('1.80', sha256='082f53530eee3f9ee84ec449eca59a77ff114250cd7daf9519679537b5b21d67')
-    version('1.74', '1372da13a83862c186aa0b6b0c9b86f5')
+    version('1.74', sha256='55ac423d5766c74236700a47838ed66bea47ba42e1d594fdd894074ba3eb0567')
 
     depends_on('perl')