Function log_filter 

pub fn log_filter(lookup: impl Fn(&str) -> Option<String>) -> String

The log filter to use, under either spelling of each setting.

Resolved rather than read straight by env_logger: Env::filter_or falls back to its default only when the variable is unset, so a blank USAGECLI_LOG would be taken as the filter instead of falling through to USAGE_LOG — which is the rule setting promises.

Precedence: trace over debug over an explicit level, and info if none of them says otherwise.