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-pypar/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class PyPypar(Package):
+    """Pypar is an efficient but easy-to-use module that allows programs written in Python to run in parallel on multiple processors and communicate using MPI."""
+    homepage = "http://code.google.com/p/pypar/"
+    url      = "https://pypar.googlecode.com/files/pypar-2.1.5_108.tgz"
+
+    version('2.1.5_108', '7a1f28327d2a3b679f9455c843d850b8', url='https://pypar.googlecode.com/files/pypar-2.1.5_108.tgz')
+    extends('python')
+    depends_on('mpi')
+
+    def install(self, spec, prefix):
+        with working_dir('source'):
+            python('setup.py', 'install', '--prefix=%s' % prefix)