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/py-scipy/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class PyScipy(Package):
+    """Scientific Library for Python."""
+    homepage = "https://pypi.python.org/pypi/scipy"
+    url      = "https://pypi.python.org/packages/source/s/scipy/scipy-0.15.0.tar.gz"
+
+    version('0.15.0', '639112f077f0aeb6d80718dc5019dc7a')
+    version('0.15.1', 'be56cd8e60591d6332aac792a5880110')
+
+    extends('python')
+    depends_on('py-nose')
+    depends_on('py-numpy')
+    depends_on('blas')
+    depends_on('lapack')
+
+    def install(self, spec, prefix):
+        python('setup.py', 'install', '--prefix=%s' % prefix)