New, cleaner package repository structure.

Package repositories now look like this:

    top-level-dir/
        repo.yaml
        packages/
            libelf/
                package.py
            mpich/
                package.py
            ...

This leaves room at the top level for additional metadata, source,
per-repo configs, indexes, etc., and it makes it easy to see that
something is a spack repo (just look for repo.yaml and packages).

var/spack/repos/builtin/packages/silo/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Silo(Package):
+    """Silo is a library for reading and writing a wide variety of scientific data to binary, disk files."""
+
+    homepage = "http://wci.llnl.gov/simulation/computer-codes/silo"
+    url      = "https://wci.llnl.gov/content/assets/docs/simulation/computer-codes/silo/silo-4.8/silo-4.8.tar.gz"
+
+    #version('4.9', 'a83eda4f06761a86726e918fc55e782a')
+    version('4.8', 'b1cbc0e7ec435eb656dc4b53a23663c9')
+
+    depends_on("hdf5@:1.8.12")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--with-hdf5=%s" %spec['hdf5'].prefix)
+
+        make()
+        make("install")