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/mock/findatender/options.go

@@ -0,0 +1,40 @@
+package mock
+
+import (
+	"log/slog"
+)
+
+// Option configures a Store, Generator or handler.
+type Option func(*options)
+
+// options holds the settings Options change.
+type options struct {
+	log *slog.Logger
+}
+
+// WithLogger makes a Store, Generator or handler log to logger instead of
+// slog.Default().
+func WithLogger(logger *slog.Logger) Option {
+	return func(opts *options) {
+		opts.log = logger
+	}
+}
+
+// applyOptions returns the settings opts make.
+func applyOptions(opts []Option) options {
+	var settings options
+	for _, opt := range opts {
+		opt(&settings)
+	}
+	return settings
+}
+
+// logger returns the logger to use, tagged with component: the given one, or
+// slog.Default(), looked up now.
+func (opts options) logger(component string) *slog.Logger {
+	log := opts.log
+	if log == nil {
+		log = slog.Default()
+	}
+	return log.With("component", component)
+}