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/libffi/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class Libffi(Package):
+    """The libffi library provides a portable, high level programming
+    interface to various calling conventions. This allows a programmer
+    to call any function specified by a call interface description at
+    run time."""
+    homepage = "https://sourceware.org/libffi/"
+    
+    version('3.2.1','83b89587607e3eb65c70d361f13bab43',url = "ftp://sourceware.org/pub/libffi/libffi-3.2.1.tar.gz")
+    #version('3.1', 'f5898b29bbfd70502831a212d9249d10',url = "ftp://sourceware.org/pub/libffi/libffi-3.1.tar.gz") # Has a bug $(lib64) instead of ${lib64} in libffi.pc
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")
+