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/isl/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Isl(Package):
+    """isl is a thread-safe C library for manipulating sets and
+    relations of integer points bounded by affine constraints."""
+    homepage = "http://isl.gforge.inria.fr"
+    url      = "http://isl.gforge.inria.fr/isl-0.14.tar.bz2"
+
+    version('0.14', 'acd347243fca5609e3df37dba47fd0bb')
+
+    depends_on("gmp")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--with-gmp-prefix=%s" % spec['gmp'].prefix)
+        make()
+        make("install")