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/libgcrypt/package.py

@@ -0,0 +1,19 @@
+from spack import *
+
+class Libgcrypt(Package):
+    """Libgcrypt is a general purpose cryptographic library based on
+       the code from GnuPG. It provides functions for all cryptographic
+       building blocks: symmetric ciphers, hash algorithms, MACs, public
+       key algorithms, large integer functions, random numbers and a lot
+       of supporting functions. """
+    homepage = "http://www.gnu.org/software/libgcrypt/"
+    url      = "ftp://ftp.gnupg.org/gcrypt/libgcrypt/libgcrypt-1.6.2.tar.bz2"
+
+    version('1.6.2', 'b54395a93cb1e57619943c082da09d5f')
+
+    depends_on("libgpg-error")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix)
+        make()
+        make("install")