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/zeromq/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class Zeromq(Package):
+    """ The ZMQ networking/concurrency library and core API """
+    homepage = "http://zguide.zeromq.org/"
+    url      = "http://download.zeromq.org/zeromq-4.1.2.tar.gz"
+
+    version('4.1.2', '159c0c56a895472f02668e692d122685')
+    version('4.1.1', '0a4b44aa085644f25c177f79dc13f253')
+    version('4.0.7', '9b46f7e7b0704b83638ef0d461fd59ab')
+    version('4.0.6', 'd47dd09ed7ae6e7fd6f9a816d7f5fdf6')
+    version('4.0.5', '73c39f5eb01b9d7eaf74a5d899f1d03d')
+
+    depends_on("libsodium")
+
+    def install(self, spec, prefix):
+        configure("--with-libsodium","--prefix=%s" % prefix)
+
+        make()
+        make("install")