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/gnutls/package.py

@@ -0,0 +1,22 @@
+from spack import *
+
+class Gnutls(Package):
+    """GnuTLS is a secure communications library implementing the SSL,
+       TLS and DTLS protocols and technologies around them. It
+       provides a simple C language application programming interface
+       (API) to access the secure communications protocols as well as
+       APIs to parse and write X.509, PKCS #12, OpenPGP and other
+       required structures. It is aimed to be portable and efficient
+       with focus on security and interoperability."""
+
+    homepage = "http://www.gnutls.org"
+    url      = "ftp://ftp.gnutls.org/gcrypt/gnutls/v3.3/gnutls-3.3.9.tar.xz"
+
+    version('3.3.9', 'ff61b77e39d09f1140ab5a9cf52c58b6')
+
+    depends_on("nettle")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")