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/zlib/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Zlib(Package):
+    """zlib is designed to be a free, general-purpose, legally unencumbered --
+       that is, not covered by any patents -- lossless data-compression library for
+       use on virtually any computer hardware and operating system.
+    """
+
+    homepage = "http://zlib.net"
+    url      = "http://zlib.net/zlib-1.2.8.tar.gz"
+
+    version('1.2.8', '44d667c142d7cda120332623eab69f40')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")