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/libgpg-error/package.py

@@ -0,0 +1,17 @@
+from spack import *
+
+class LibgpgError(Package):
+    """Libgpg-error is a small library that defines common error
+       values for all GnuPG components. Among these are GPG, GPGSM,
+       GPGME, GPG-Agent, libgcrypt, Libksba, DirMngr, Pinentry,
+       SmartCard Daemon and possibly more in the future. """
+
+    homepage = "https://www.gnupg.org/related_software/libgpg-error"
+    url      = "ftp://ftp.gnupg.org/gcrypt/libgpg-error/libgpg-error-1.18.tar.bz2"
+
+    version('1.18', '12312802d2065774b787cbfc22cc04e9')
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")