Logging: structured slog messages throughout, and WithLogger options so callers choose the logger

Log messages are lower-case event names with their details as key/value
attributes ("error" for errors), in place of formatted strings.

Loggable package take a WithLogger option as part of their "New*" call,
defaulting to slog.Default(). Each tags its lines with its `component`.

Some debug loggers print directly rather than using slog when in
terminal logging mode.

internal/cli/options.go

@@ -65,7 +65,7 @@ func (o *Options) RunSetup(
 		// Check if the logging should be switched to file
 		if !terminalLog {
 			if len(conf.Logging.File) == 0 {
-				slog.Warn("No log file configured, remaining in console logging mode!")
+				slog.Warn("no log file configured; logging to the terminal")
 			} else if err := logging.LogToFile(conf.Logging.File); err != nil {
 				return nil, fmt.Errorf("opening log file: %w", err)
 			}