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/nasm/package.py

@@ -0,0 +1,14 @@
+from spack import *
+
+class Nasm(Package):
+    """NASM (Netwide Assembler) is an 80x86 assembler designed for
+       portability and modularity. It includes a disassembler as well."""
+    homepage = "http://www.nasm.us"
+    url      = "http://www.nasm.us/pub/nasm/releasebuilds/2.11.06/nasm-2.11.06.tar.xz"
+
+    version('2.11.06', '2b958e9f5d200641e6fc9564977aecc5')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")