Use ENV_JUPYTER_PATH as default system path for installing kernels

[jdemeyer]

When installing kernels using install_kernel_spec(), the default directory should be what distutils uses to install data files, which is not /usr/local/share but some directory based on sys.prefix. So, we should use ENV_JUPYTER_PATH instead of SYSTEM_JUPYTER_PATH for the system installation directory.

Apply this instead of jupyter/jupyter_core#61

[minrk]

sys.prefix is not a reliable indicator of where things should go, and it has the wrong value is many common cases as discussed in jupyter/jupyter_core#61.

[minrk]

That said, we could accept this change and push a patch to core that changes how ENV_JUPYTER_PATH is determined to actually get the information from distutils instead of the unreliable sys.prefix. I never like interacting with distutils, but it might be the right source of this information.

[pkgw]

If this is applied, a similar change should be made to _get_nbext_dir in notebook/nbextensions.py.

FWIW, in the Anaconda distribution, sys.prefix and friends have eminently reasonable values.

[minrk]

@pkgw yes, in envs sys.prefix should always be reliable. But that's not everywhere, and sys.prefix is definitely wrong in very common cases, so we would need to special-case those if we end up going this way.

[jdemeyer]

I think this can be closed now.