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/perl-carp-clan/package.py

@@ -12,7 +12,7 @@ class PerlCarpClan(PerlPackage):
     homepage = "http://search.cpan.org/~kentnl/Carp-Clan-6.06/lib/Carp/Clan.pod"
     url      = "http://search.cpan.org/CPAN/authors/id/K/KE/KENTNL/Carp-Clan-6.06.tar.gz"
 
-    version('6.06', 'c562a35c48f43665fab735cdc7fe3cb2')
+    version('6.06', sha256='ea4ac8f611354756d43cb369880032901e9cc4cc7e0bebb7b647186dac00c9d4')
 
     depends_on('perl-test-exception', type=('build', 'run'))
     depends_on('perl-sub-uplevel', type=('build', 'run'))