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/pidx/package.py

@@ -0,0 +1,21 @@
+from spack import *
+
+class Pidx(Package):
+    """PIDX Parallel I/O Library.
+
+    PIDX is an efficient parallel I/O library that reads and writes
+    multiresolution IDX data files.
+    """
+
+    homepage = "http://www.cedmav.com/pidx"
+
+    version('1.0', git='https://github.com/sci-visus/PIDX.git',
+            commit='6afa1cf71d1c41263296dc049c8fabaf73c296da')
+
+    depends_on("mpi")
+
+    def install(self, spec, prefix):
+        with working_dir('spack-build', create=True):
+            cmake('..', *std_cmake_args)
+            make()
+            make("install")