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/petsc/package.py

@@ -0,0 +1,40 @@
+from spack import *
+
+class Petsc(Package):
+    """PETSc is a suite of data structures and routines for the
+       scalable (parallel) solution of scientific applications modeled by
+       partial differential equations."""
+
+    homepage = "http://www.mcs.anl.gov/petsc/index.html"
+    url      = "http://ftp.mcs.anl.gov/pub/petsc/release-snapshots/petsc-3.5.3.tar.gz"
+
+    version('3.5.3', 'd4fd2734661e89f18ac6014b5dd1ef2f')
+    version('3.5.2', 'ad170802b3b058b5deb9cd1f968e7e13')
+    version('3.5.1', 'a557e029711ebf425544e117ffa44d8f')
+
+    depends_on("boost")
+    depends_on("blas")
+    depends_on("lapack")
+    depends_on("hypre")
+    depends_on("parmetis")
+    depends_on("metis")
+    depends_on("hdf5")
+    depends_on("mpi")
+
+    def install(self, spec, prefix):
+        configure("--prefix=%s" % prefix,
+                  "CC=cc",
+                  "CXX=c++",
+                  "FC=f90",
+                  "--with-blas-lib=%s/libblas.a"     % spec['blas'].prefix.lib,
+                  "--with-lapack-lib=%s/liblapack.a" % spec['lapack'].prefix.lib,
+                  "--with-boost-dir=%s"              % spec['boost'].prefix,
+                  "--with-hypre-dir=%s"              % spec['hypre'].prefix,
+                  "--with-parmetis-dir=%s"           % spec['parmetis'].prefix,
+                  "--with-metis-dir=%s"              % spec['metis'].prefix,
+                  "--with-hdf5-dir=%s"               % spec['hdf5'].prefix,
+                  "--with-shared-libraries=0")
+
+        # PETSc has its own way of doing parallel make.
+        make('MAKE_NP=%s' % make_jobs, parallel=False)
+        make("install")