Wheelhouse doesn't work for packages using setup_requires

[johnsca]

Originally raised in juju-solutions/layer-apache-hadoop-namenode#7

This causes the charm to still hit pypi during install and thus breaks network-restricted deployments. It doesn't affect most packages, but could potentially affect any of them.

[johnsca]

Informational links from original issue:

• pypa/pip#410
• https://pip.pypa.io/en/latest/reference/pip_install/#controlling-setup-requires

The latter link is a possible work-around, but would still require libraries referenced by setup_requires to be manually enumerated in a layer's wheelhouse.txt unless we can come up with some way to automatically discover them during build.

[johnsca]

This was addressed in juju-solutions/layer-basic#51