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/git/package.py

@@ -0,0 +1,27 @@
+from spack import *
+
+class Git(Package):
+    """Git is a free and open source distributed version control
+       system designed to handle everything from small to very large
+       projects with speed and efficiency."""
+    homepage = "http://git-scm.com"
+    url      = "https://www.kernel.org/pub/software/scm/git/git-2.2.1.tar.xz"
+
+    version('2.2.1', '43e01f9d96ba8c11611e0eef0d9f9f28')
+
+    # Use system openssl.
+    # depends_on("openssl")
+
+    # Use system perl for now.
+    # depends_on("perl")
+    # depends_on("pcre")
+
+    depends_on("zlib")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "--without-pcre",
+                  "--without-python")
+
+        make()
+        make("install")