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/bison/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Bison(Package):
+    """Bison is a general-purpose parser generator that converts 
+    an annotated context-free grammar into a deterministic LR or 
+    generalized LR (GLR) parser employing LALR(1) parser tables."""
+
+    homepage = "http://www.gnu.org/software/bison/"
+    url      = "http://ftp.gnu.org/gnu/bison/bison-3.0.tar.gz"
+
+    version('3.0.4', 'a586e11cd4aff49c3ff6d3b6a4c9ccf8')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")