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/nettle/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Nettle(Package):
+    """The Nettle package contains the low-level cryptographic library
+    that is designed to fit easily in many contexts."""
+
+    homepage = "http://www.example.com"
+    url      = "http://ftp.gnu.org/gnu/nettle/nettle-2.7.1.tar.gz"
+
+    version('2.7', '2caa1bd667c35db71becb93c5d89737f')
+
+    depends_on('gmp')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")