Make the trampoline code more robust & clean.

Previous version polluted the `vtkmodules` package by adding all
attributes from `vtkmodules.all` to it. This new version doesn't do
that.

Wrapping/Python/vtk.py

 """This is the vtk module."""
-import sys, importlib
-
-vtkmodules = importlib.import_module("vtkmodules")
-vtkmodules_all = importlib.import_module("vtkmodules.all")
-# merge components from `all` to `vtkmodules` package.
-for key in dir(vtkmodules_all):
-    if not hasattr(vtkmodules, key):
-        setattr(vtkmodules, key, getattr(vtkmodules_all, key))
-
-# replace ourselves with the `vtkmodules` package. This is essential
-# to ensure the importing of other modules/packages from `vtkmodules`
-# works seamlessly when accessed via `vtk` pseudo-package.
-sys.modules[__name__] = vtkmodules
+import sys
+
+if sys.version_info < (3,4):
+    # imp is deprecated in 3.4
+    import imp, importlib
+
+    # import vtkmodules package.
+    vtkmodules_m = importlib.import_module('vtkmodules')
+
+    # import vtkmodules.all
+    all_m = importlib.import_module('vtkmodules.all')
+
+    # create a clone of the `vtkmodules.all` module.
+    vtk_m = imp.new_module(__name__)
+    for key in dir(all_m):
+        if not hasattr(vtk_m, key):
+            setattr(vtk_m, key, getattr(all_m, key))
+
+    # make the clone of `vtkmodules.all` act as a package at the same location
+    # as vtkmodules. This ensures that importing modules from within the vtkmodules package
+    # continues to work.
+    vtk_m.__path__ = vtkmodules_m.__path__
+    # replace old `vtk` module with this new package.
+    sys.modules[__name__] = vtk_m
+
+else:
+    import importlib.util
+    # import vtkmodules.all
+    all_spec = importlib.util.find_spec('vtkmodules.all')
+    all_m = importlib.util.module_from_spec(all_spec)
+    all_spec.loader.exec_module(all_m)
+
+    # import vtkmodules
+    vtkmodules_spec = importlib.util.find_spec('vtkmodules')
+    vtkmodules_m = importlib.util.module_from_spec(vtkmodules_spec)
+    vtkmodules_spec.loader.exec_module(vtkmodules_m)
+
+    # make vtkmodules.all act as the vtkmodules package to support importing
+    # other modules from vtkmodules package via `vtk`.
+    all_m.__path__ = vtkmodules_m.__path__
+
+    del all_spec, vtkmodules_spec
+
+    # replace old `vtk` module with the `all` package.
+    sys.modules[__name__] = all_m