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/fish/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Fish(Package):
+    """fish is a smart and user-friendly command line shell for OS X, Linux, and
+    the rest of the family.
+    """
+
+    homepage = "http://fishshell.com/"
+    url      = "http://fishshell.com/files/2.2.0/fish-2.2.0.tar.gz"
+    list_url = homepage
+
+    version('2.2.0', 'a76339fd14ce2ec229283c53e805faac48c3e99d9e3ede9d82c0554acfc7b77a')
+
+    def install(self, spec, prefix):
+        configure('--prefix=%s' % prefix)
+
+        make()
+        make("install")