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/libuuid/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class Libuuid(Package):
+    """Portable uuid C library"""
+    # FIXME: add a proper url for your package's homepage here.
+    homepage = "http://sourceforge.net/projects/libuuid/"
+    url      = "http://downloads.sourceforge.net/project/libuuid/libuuid-1.0.3.tar.gz?r=http%3A%2F%2Fsourceforge.net%2Fprojects%2Flibuuid%2F&ts=1433881396&use_mirror=iweb"
+
+    version('1.0.3', 'd44d866d06286c08ba0846aba1086d68')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        # FIXME: Add logic to build and install here
+        make()
+        make("install")