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/libpciaccess/package.py

@@ -0,0 +1,21 @@
+from spack import *
+
+class Libpciaccess(Package):
+    """Generic PCI access library."""
+
+    homepage = "http://cgit.freedesktop.org/xorg/lib/libpciaccess/"
+    url      = "http://cgit.freedesktop.org/xorg/lib/libpciaccess/"
+
+    version('0.13.4', git='http://anongit.freedesktop.org/git/xorg/lib/libpciaccess.git',
+            tag='libpciaccess-0.13.4')
+
+    depends_on('autoconf')
+    depends_on('libtool')
+
+    def install(self, spec, prefix):
+        from subprocess import call
+        call(["./autogen.sh"])
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")