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/libtiff/package.py

@@ -0,0 +1,18 @@
+from spack import *
+
+class Libtiff(Package):
+    """libtiff graphics format library"""
+    homepage = "http://www.remotesensing.org/libtiff/"
+    url      = "http://download.osgeo.org/libtiff/tiff-4.0.3.tar.gz"
+
+    version('4.0.3', '051c1068e6a0627f461948c365290410')
+
+    depends_on('jpeg')
+    depends_on('zlib')
+    depends_on('xz')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")