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/ghostscript/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Ghostscript(Package):
+    """an interpreter for the PostScript language and for PDF. """
+    homepage = "http://ghostscript.com/"
+    url      = "http://downloads.ghostscript.com/public/ghostscript-9.16.tar.gz"
+
+    version('9.16', '829319325bbdb83f5c81379a8f86f38f')
+
+    parallel = False
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" %prefix, "--enable-shared")
+
+        make()
+        make("install")
+