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/vtk/package.py

@@ -0,0 +1,40 @@
+from spack import *
+
+class Vtk(Package):
+    """The Visualization Toolkit (VTK) is an open-source, freely
+    available software system for 3D computer graphics, image
+    processing and visualization. """
+    homepage = "http://www.vtk.org"
+    url      = "http://www.vtk.org/files/release/6.1/VTK-6.1.0.tar.gz"
+
+    version('6.1.0', '25e4dfb3bad778722dcaec80cd5dab7d')
+
+    depends_on("qt")
+
+    def install(self, spec, prefix):
+        with working_dir('spack-build', create=True):
+            cmake_args = [
+                "..",
+                "-DBUILD_SHARED_LIBS=ON",
+                # Disable wrappers for other languages.
+                "-DVTK_WRAP_PYTHON=OFF",
+                "-DVTK_WRAP_JAVA=OFF",
+                "-DVTK_WRAP_TCL=OFF"]
+            cmake_args.extend(std_cmake_args)
+
+            # Enable Qt support here.
+            cmake_args.extend([
+                "-DQT_QMAKE_EXECUTABLE:PATH=%s/qmake" % spec['qt'].prefix.bin,
+                "-DVTK_Group_Qt:BOOL=ON",
+                # Ignore webkit because it's hard to build w/Qt
+                "-DVTK_Group_Qt=OFF",
+                "-DModule_vtkGUISupportQt:BOOL=ON",
+                "-DModule_vtkGUISupportQtOpenGL:BOOL=ON"
+                ])
+
+            if spec['qt'].satisfies('@5'):
+                cmake_args.append("-DVTK_QT_VERSION:STRING=5")
+
+            cmake(*cmake_args)
+            make()
+            make("install")