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/the_silver_searcher/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class TheSilverSearcher(Package):
+    """Fast recursive grep alternative"""
+    homepage = "http://geoff.greer.fm/ag/"
+    url      = "http://geoff.greer.fm/ag/releases/the_silver_searcher-0.30.0.tar.gz"
+
+    version('0.30.0', '95e2e7859fab1156c835aff7413481db')
+
+    depends_on('pcre')
+    depends_on('xz')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")