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/coreutils/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Coreutils(Package):
+    """The GNU Core Utilities are the basic file, shell and text
+       manipulation utilities of the GNU operating system.  These are
+       the core utilities which are expected to exist on every
+       operating system.
+    """
+    homepage = "http://www.gnu.org/software/coreutils/"
+    url      = "http://ftp.gnu.org/gnu/coreutils/coreutils-8.23.tar.xz"
+
+    version('8.23', 'abed135279f87ad6762ce57ff6d89c41')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")