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/zsh/package.py

@@ -0,0 +1,16 @@
+from spack import *
+
+class Zsh(Package):
+    """ The ZSH shell """
+    homepage = "http://www.zsh.org"
+    url      = "http://www.zsh.org/pub/zsh-5.0.8.tar.bz2"
+
+    version('5.0.8', 'e6759e8dd7b714d624feffd0a73ba0fe')
+
+    depends_on("pcre")
+
+    def install(self, spec, prefix):
+        configure('--prefix=%s' % prefix)
+
+        make()
+        make("install")