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/libjpeg-turbo/package.py

@@ -0,0 +1,20 @@
+from spack import *
+
+class LibjpegTurbo(Package):
+    """libjpeg-turbo is a fork of the original IJG libjpeg which uses
+       SIMD to accelerate baseline JPEG compression and
+       decompression. libjpeg is a library that implements JPEG image
+       encoding, decoding and transcoding."""
+    homepage = "http://libjpeg-turbo.virtualgl.org"
+    url      = "http://downloads.sourceforge.net/libjpeg-turbo/libjpeg-turbo-1.3.1.tar.gz"
+
+    version('1.3.1', '2c3a68129dac443a72815ff5bb374b05')
+
+    # Can use either of these.
+    depends_on("yasm")
+    depends_on("nasm")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")