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/ppl/package.py

@@ -0,0 +1,28 @@
+from spack import *
+
+class Ppl(Package):
+    """The Parma Polyhedra Library (PPL) provides numerical
+    abstractions especially targeted at applications in the field of
+    analysis and verification of complex systems. These abstractions
+    include convex polyhedra, some special classes of polyhedra shapes
+    that offer interesting complexity/precision tradeoffs, and grids
+    which represent regularly spaced points that satisfy a set of
+    linear congruence relations. The library also supports finite
+    powersets and products of polyhedra and grids, a mixed integer
+    linear programming problem solver using an exact-arithmetic
+    version of the simplex algorithm, a parametric integer programming
+    solver, and primitives for termination analysis via the automatic
+    synthesis of linear ranking functions."""
+
+    homepage = "http://bugseng.com/products/ppl/"
+    url      = "http://bugseng.com/products/ppl/download/ftp/releases/1.1/ppl-1.1.tar.gz"
+
+    version('1.1', '4f2422c0ef3f409707af32108deb30a7')
+
+    depends_on("gmp")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--with-gmp=%s" % spec['gmp'].prefix)
+        make()
+        make("install")