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/libsodium/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Libsodium(Package):
+    """Sodium is a modern, easy-to-use software library for encryption,
+    decryption, signatures, password hashing and more."""
+    homepage = "https://download.libsodium.org/doc/"
+    url      = "https://download.libsodium.org/libsodium/releases/libsodium-1.0.3.tar.gz"
+
+    version('1.0.3', 'b3bcc98e34d3250f55ae196822307fab')
+    version('1.0.2', 'dc40eb23e293448c6fc908757738003f')
+    version('1.0.1', '9a221b49fba7281ceaaf5e278d0f4430')
+    version('1.0.0', '3093dabe4e038d09f0d150cef064b2f7')
+    version('0.7.1', 'c224fe3923d1dcfe418c65c8a7246316')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")