add logging to resolver

[dhellmann]

Implement the reporter API used by resolvelib so that the resolver can
report progress and explain why candidates are rejected.

Fixes #548

[dhellmann]

Some sample output from a run downstream

pyproject-metadata: looking for candidates for <Requirement('pyproject-metadata>=0.7.1')>
pyproject-metadata: selecting <pyproject-metadata==0.9.0>
pyproject-metadata: successfully resolved <Requirement('pyproject-metadata>=0.7.1')>
pyproject-metadata: new build-system dependency pyproject-metadata>=0.7.1 resolves to 0.9.0
pyproject-metadata: checking if wheel was already uploaded to https://gitlab.com/api/v4/projects/66357902/packages/pypi/simple/
pyproject-metadata: looking for candidates for <Requirement('pyproject-metadata==0.9.0')>
pyproject-metadata: selecting <pyproject-metadata==0.9.0>
pyproject-metadata: successfully resolved <Requirement('pyproject-metadata==0.9.0')>
pyproject-metadata: has expected build tag (1, '')
pyproject-metadata: have existing wheel /work/bootstrap-output.cuda-ubi9/wheels-repo/downloads/pyproject_metadata-0.9.0-1-py3-none-any.whl
pyproject-metadata: found built wheel on cache server

[rd4398]

Looks good to me!

[rd4398]

Looks good