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/xz/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class Xz(Package):
+    """XZ Utils is free general-purpose data compression software with
+       high compression ratio. XZ Utils were written for POSIX-like
+       systems, but also work on some not-so-POSIX systems. XZ Utils are
+       the successor to LZMA Utils."""
+    homepage = "http://tukaani.org/xz/"
+    url      = "http://tukaani.org/xz/xz-5.2.0.tar.bz2"
+
+    version('5.2.0', '867cc8611760240ebf3440bd6e170bb9',
+            url = 'http://tukaani.org/xz/xz-5.2.0.tar.bz2')
+    version('5.2.2', 'f90c9a0c8b259aee2234c4e0d7fd70af',
+            url = 'http://tukaani.org/xz/xz-5.2.2.tar.bz2')
+   
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
+