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/hdf5/package.py

@@ -0,0 +1,42 @@
+from spack import *
+
+class Hdf5(Package):
+    """HDF5 is a data model, library, and file format for storing and managing
+       data. It supports an unlimited variety of datatypes, and is designed for
+       flexible and efficient I/O and for high volume and complex data.
+    """
+
+    homepage = "http://www.hdfgroup.org/HDF5/"
+    url      = "http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-1.8.13/src/hdf5-1.8.13.tar.gz"
+    list_url = "http://www.hdfgroup.org/ftp/HDF5/releases"
+    list_depth = 3
+    
+    version('1.8.15', '03cccb5b33dbe975fdcd8ae9dc021f24')
+    version('1.8.13', 'c03426e9e77d7766944654280b467289')
+
+    depends_on("mpi")
+    depends_on("zlib")
+
+    # TODO: currently hard-coded to use OpenMPI
+    def install(self, spec, prefix):
+
+        configure(
+            "--prefix=%s" % prefix,
+            "--with-zlib=%s" % spec['zlib'].prefix,
+            "--enable-parallel",
+            "--enable-shared",
+            "CC=%s" % spec['mpich'].prefix.bin + "/mpicc",
+            "CXX=%s" % spec['mpich'].prefix.bin + "/mpic++")
+
+        make()
+        make("install")
+
+    def url_for_version(self, version):
+        v = str(version)
+
+        if version == Version("1.2.2"):
+            return "http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-" + v + ".tar.gz"
+        elif version < Version("1.7"):
+            return "http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-" + version.up_to(2) + "/hdf5-" + v + ".tar.gz"
+        else:
+            return "http://www.hdfgroup.org/ftp/HDF5/releases/hdf5-" + v + "/src/hdf5-" + v + ".tar.gz"