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/matio/package.py

@@ -0,0 +1,15 @@
+from spack import *
+
+
+class Matio(Package):
+    """matio is an C library for reading and writing Matlab MAT files"""
+    homepage = "http://sourceforge.net/projects/matio/"
+    url = "http://downloads.sourceforge.net/project/matio/matio/1.5.2/matio-1.5.2.tar.gz"
+
+    version('1.5.2', '85b007b99916c63791f28398f6a4c6f1')
+
+    def install(self, spec, prefix):
+        configure('--prefix=%s' % prefix)
+
+        make()
+        make("install")