Consolidate download progress reporting

Bazel downloads files both as part of repository rule execution and when fetching results from a remote cache. We should ensure that the way download progress is reported on the command line is consistent and contains useful information without being overly verbose.

Start by tracing the two download paths named in the issue: repository rule execution and remote-cache result fetching. Compare their command-line progress output and define a consistent, useful format that avoids excess verbosity; done means both paths report downloads in the same way.

Written by the indexing model from the issue text.