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/autoconf/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class Autoconf(Package):
+    """Autoconf -- system configuration part of autotools"""
+    homepage = "https://www.gnu.org/software/autoconf/"
+    url      = "http://ftp.gnu.org/gnu/autoconf/autoconf-2.69.tar.gz"
+
+    version('2.69', '82d05e03b93e45f5a39b828dc9c6c29b')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+
+        make()
+        make("install")