ImportError from wheel installation

Hello - thank you so much for the energy that you are investing in providing a wheel installation for VTK.

I have successfully installed the software, but am running into errors on import.

When installing from conda, I get the following error:

```
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
/srv/conda/envs/notebook/lib/python3.7/site-packages/vtkmodules/vtkRenderingOpenGL2.py in
4 # use relative import for installed modules
----> 5 from .vtkRenderingOpenGL2Python import *
6 except ImportError:

ImportError: libGL.so.1: cannot open shared object file: No such file or directory

During handling of the above exception, another exception occurred:

ModuleNotFoundError Traceback (most recent call last)
in
----> 1 import vtk

/srv/conda/envs/notebook/lib/python3.7/site-packages/vtk.py in
30 all_spec = importlib.util.find_spec('vtkmodules.all')
31 all_m = importlib.util.module_from_spec(all_spec)
---> 32 all_spec.loader.exec_module(all_m)
33
34 # import vtkmodules

/srv/conda/envs/notebook/lib/python3.7/site-packages/vtkmodules/all.py in
31 from .vtkIOXMLParser import *
32 from .vtkDomainsChemistry import *
---> 33 from .vtkRenderingOpenGL2 import *
34 from .vtkDomainsChemistryOpenGL2 import *
35 from .vtkIOXML import *

/srv/conda/envs/notebook/lib/python3.7/site-packages/vtkmodules/vtkRenderingOpenGL2.py in
7 # during build and testing, the modules will be elsewhere,
8 # e.g. in lib directory or Release/Debug config directories
----> 9 from vtkRenderingOpenGL2Python import *

ModuleNotFoundError: No module named 'vtkRenderingOpenGL2Python'
```

When installing with pip, I instead get the following error:

```
---------------------------------------------------------------------------
ImportError Traceback (most recent call last)
/srv/conda/envs/notebook/lib/python3.7/site-packages/vtk/vtkOpenGLKit.py in
4 # use relative import for installed modules
----> 5 from .vtkOpenGLKitPython import *
6 except ImportError:

ImportError: libGL.so.1: cannot open shared object file: No such file or directory

During handling of the above exception, another exception occurred:

ModuleNotFoundError Traceback (most recent call last)
in
----> 1 import vtk

/srv/conda/envs/notebook/lib/python3.7/site-packages/vtk/__init__.py in
10 from .vtkRenderingKit import *
11 from .vtkIOKit import *
---> 12 from .vtkOpenGLKit import *
13 from .vtkParallelKit import *
14 from .vtkWrappingKit import *

/srv/conda/envs/notebook/lib/python3.7/site-packages/vtk/vtkOpenGLKit.py in
7 # during build and testing, the modules will be elsewhere,
8 # e.g. in lib directory or Release/Debug config directories
----> 9 from vtkOpenGLKitPython import *

ModuleNotFoundError: No module named 'vtkOpenGLKitPython'
```

Note also that this is happening on a headless system (through a Jupyter notebook, running on a JupyterHub installed on EKS), which may be pertinent information.

Thanks!

Start by reproducing `import vtk` in the reported headless JupyterHub environment on EKS using both conda and pip installations. Inspect the import paths in `vtkmodules/vtkRenderingOpenGL2.py` and `vtk/vtkOpenGLKit.py`; done means the package imports successfully in the reported headless setup without falling through to missing-module errors.

索引モデルが issue の本文から書いたものです。