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/hpx/package.py

@@ -18,7 +18,7 @@ class Hpx(CMakePackage, CudaPackage):
     version('1.2.0', sha256='20942314bd90064d9775f63b0e58a8ea146af5260a4c84d0854f9f968077c170')
     version('1.1.0', sha256='1f28bbe58d8f0da600d60c3a74a644d75ac777b20a018a5c1c6030a470e8a1c9')
 
-    version('1.0.0', '4983e7c6402417ec794d40343e36e417', url='http://stellar.cct.lsu.edu/files/hpx_1.0.0')
+    version('1.0.0', md5='4983e7c6402417ec794d40343e36e417', url='http://stellar.cct.lsu.edu/files/hpx_1.0.0')
 
     variant('cxxstd',
             default='17',