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/intel-gpu-tools/package.py

@@ -20,7 +20,7 @@ class IntelGpuTools(AutotoolsPackage):
     homepage = "https://cgit.freedesktop.org/xorg/app/intel-gpu-tools/"
     url      = "https://www.x.org/archive/individual/app/intel-gpu-tools-1.16.tar.gz"
 
-    version('1.16', '3996f10fc86a28ec59e1cf7b227dad78')
+    version('1.16', sha256='4874e6e7704c8d315deaf5b44cc9467ea5e502c7f816470a4a28827fcb34643f')
 
     depends_on('libdrm@2.4.64:')
     depends_on('libpciaccess@0.10:', when=(sys.platform != 'darwin'))