Installing pip/apt packages in only-proxy environment fails

If the juju model config has only the juju-http-proxy/juju-https-proxy settings the layer basic tries to install python_packages via pip defined in layer.yaml and fails (hangs).

layer.yaml:
options:
basic:
packages:
# Install "juju" Python package build dependencies.
- libssl-dev
- libffi-dev
python_packages:
# do not put it in wheelhouse.txt, since its deps need compilaton
- juju==2.7.1
use_venv: false

For example the following config is set:

uju-http-proxy model http://squid.internal:3128
juju-https-proxy model http://squid.internal:3128

The following options are blank:
http-proxy default ""
https-proxy default ""

If you configure http-proxy and https-proxy on the model the pip installation succeeds (global proxy settings, which we want to stay away from).

All juju charms should be able to work with juju-http(s)-proxy options.

Start with the layer.yaml package and python_packages configuration, then trace how the layer handles juju-http-proxy and juju-https-proxy during pip and apt installation. Compare that path with the global http-proxy and https-proxy settings and reproduce the hang; done means package installation succeeds using only the model-specific proxy options.

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