Add debug logging

https://github.com/bazelbuild/bazelisk/issues/743 is a recent example of when I needed to examine GCS requests, which usually means adding lots of print statements.

We should add debug logging via https://github.com/golang/glog so that similar problems can be debugged easier.

Start by inspecting the Bazelisk code path that makes GCS requests and review the golang/glog integration described in the issue. Done means debug logging makes these requests easier to examine without adding print statements; the issue names no specific files or tests.

Written by the indexing model from the issue text.