Using --resolver-version=pip-2020-resolver yields poor dependency conflict information.

[jsirois]

For example, using the legacy resolver, Pex does postprocessing of successful pip resolves to detect conflicts:

$ python3.6 -mpex --resolver-version=pip-legacy-resolver wheel==0.33.6 pantsbuild.pants==1.24.0.dev2
Failed to resolve compatible distributions:
1: pantsbuild.pants==1.24.0.dev2 requires wheel==0.31.1 but wheel 0.33.6 was resolved

When using the pip-2020-resolver Pex doesn't need to post-process (in fact can't post-process!) since Pip fails fast. Unfortunately though, its equivalent conflict information is hidden in stdout, which we suppress to hide other noise:

$ python3.6 -mpex --resolver-version=pip-2020-resolver wheel==0.33.6 pantsbuild.pants==1.24.0.dev2
ERROR: Cannot install pantsbuild-pants==1.24.0.dev2 and wheel==0.33.6 because these package versions have conflicting dependencies.
ERROR: ResolutionImpossible: for help visit https://pip.pypa.io/en/latest/user_guide/#fixing-conflicting-dependencies
pid 50027 -> /usr/bin/python3.6 /home/jsirois/.pex/pip.pex/38e9593b9e9184c3f2ffb67ab963128a7fe63808 --disable-pip-version-check --no-python-version-warning --exists-action a --isolated -q --cache-dir /home/jsirois/.pex download --dest /tmp/tmpel5zlwiq/usr.bin.python3.6 wheel==0.33.6 pantsbuild.pants==1.24.0.dev2 --index-url https://pypi.org/simple --retries 5 --timeout 15 exited with 1 and STDERR:
None

For reference, that stdout info looks like:

The conflict is caused by:
    The user requested wheel==0.33.6
    pantsbuild-pants 1.24.0.dev2 depends on wheel==0.31.1

To fix this you could try to:
1. loosen the range of package versions you've specified
2. remove package versions to allow pip attempt to solve the dependency conflict

I've filed pypa/pip#9420 to surface that information to stderr, but in the meantime it seems we'll need to pluck this information from the suppressed stdout to surface it. It appears the global --log argument to pip does not have a noticeable negative performance implication; so using that and then postprocessing the log file to find the information and display it is the best we can do pending a fix for pypa/pip#9420.