Pex should trust any host passed via --index or --find-links.

[jsirois]

The underlying pip calls don't assume this and error out, e.g.:

E   	WARNING: The repository located at www.antlr3.org is not a trusted or secure host and is being ignored. If this repository is available via HTTPS we recommend you use HTTPS instead, otherwise you may silence this warning and allow it anyway with '--trusted-host www.antlr3.org'.
E   	ERROR: Could not find a version that satisfies the requirement antlr_python_runtime==3.1.3 (from versions: none)
E   	ERROR: No matching distribution found for antlr_python_runtime==3.1.3
...
E   	Exception caught: (pex.resolver.Unsatisfiable)
...
E   	  File "/b/f/w/pants/backend/python/subsystems/pex_build_util.py", line 278, in _resolve_multi
E   	    allow_prereleases=python_setup.resolver_allow_prereleases)
E   	  File "/b/f/w/pex_root/install/pex-2.0.2-py2.py3-none-any.whl.b89b057c31d08e6bebdb4531720b75237c56588e/pex-2.0.2-py2.py3-none-any.whl/pex/resolver.py", line 195, in resolve
E   	    raise Unsatisfiable(str(e))
E   	
E   	Exception message: Executing /usr/bin/python2.7 -s -m pip --disable-pip-version-check --isolated -q --cache-dir /home/nobody/.cache/pants/python_cache/requirements/CPython-2.7.12 download --dest /tmp/tmpj2dt894x/resolved --index-url https://pypi.org/simple/ --find-links http://www.antlr3.org/download/Python/ antlr_python_runtime==3.1.3 failed with 1

[jsirois]

Related to #803 in that --trusted-host is a pip network option, but I can't see a reason to expose this directly vs assume it from indexes / find_links.