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/handler.go

@@ -5,7 +5,6 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
-	"log"
 	"net/http"
 	"net/url"
 	"strconv"
@@ -27,17 +26,20 @@ type server struct {
 	store   *Store
 	baseURL string
 	loc     *time.Location // zone for updatedFrom/updatedTo without an offset; nil means UTC
+	opts    options
 }
 
 // NewHandler serves release packages from store. baseURL is the public base
 // URL used in each package's uri and links.next. loc is the time zone that
 // updatedFrom and updatedTo are read in when they carry no offset, as the
 // documented yyyy-MM-ddTHH:mm:ss format doesn't (Europe/London for FTS).
-// Stored release dates are UTC; comparisons are on absolute instants.
+// Stored release dates are UTC; comparisons are on absolute instants. It
+// logs to slog.Default() unless given WithLogger.
 func NewHandler(
-	store *Store, baseURL string, loc *time.Location,
+	store *Store, baseURL string, loc *time.Location, opts ...Option,
 ) http.HandlerFunc {
-	return (&server{store: store, baseURL: baseURL, loc: loc}).handle
+	s := &server{store: store, baseURL: baseURL, loc: loc, opts: applyOptions(opts)}
+	return s.handle
 }
 
 // encodeCursor makes the opaque cursor for a next link that resumes at store
@@ -134,7 +136,7 @@ func (s *server) handle(w http.ResponseWriter, req *http.Request) {
 	enc.SetIndent("", "  ")
 	enc.SetEscapeHTML(false)
 	if err := enc.Encode(pkg); err != nil {
-		log.Printf("handler: writing response: %v", err)
+		s.opts.logger("handler").Warn("writing response failed", "error", err)
 	}
 }