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/util-linux/package.py

@@ -0,0 +1,20 @@
+from spack import *
+import os
+
+class UtilLinux(Package):
+    """Util-linux is a suite of essential utilities for any Linux system."""
+
+    homepage = "http://freecode.com/projects/util-linux"
+    url      = "https://www.kernel.org/pub/linux/utils/util-linux/v2.25/util-linux-2.25.tar.gz"
+
+    version('2.25', 'f6d7fc6952ec69c4dc62c8d7c59c1d57')
+
+    depends_on("python@2.7:")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix, 
+                "PKG_CONFIG_PATH=%s/pkgconfig" % spec['python'].prefix.lib,
+                "--disable-use-tty-group")
+
+        make()
+        make("install")