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/libxft/package.py

@@ -16,7 +16,7 @@ class Libxft(AutotoolsPackage):
     homepage = "http://cgit.freedesktop.org/xorg/lib/libXft"
     url      = "https://www.x.org/archive/individual/lib/libXft-2.3.2.tar.gz"
 
-    version('2.3.2', '3a2c1ce2641817dace55cd2bfe10b0f0')
+    version('2.3.2', sha256='26cdddcc70b187833cbe9dc54df1864ba4c03a7175b2ca9276de9f05dce74507')
 
     depends_on('freetype@2.1.6:')
     depends_on('fontconfig@2.5.92:')