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/libarchive/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class Libarchive(Package):
+    """libarchive: C library and command-line tools for reading and
+       writing tar, cpio, zip, ISO, and other archive formats."""
+    homepage = "http://www.libarchive.org"
+    url      = "http://www.libarchive.org/downloads/libarchive-3.1.2.tar.gz"
+
+    version('3.1.2', 'efad5a503f66329bb9d2f4308b5de98a')
+    version('3.1.1', '1f3d883daf7161a0065e42a15bbf168f')
+    version('3.1.0', '095a287bb1fd687ab50c85955692bf3a')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")