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/config/load_test.go

@@ -93,7 +93,7 @@ func TestInitNoConfigFile(t *testing.T) {
 	if err := Init("", &conf); err != nil {
 		t.Fatal(err)
 	}
-	if !strings.Contains(logs.String(), "No config file found") {
+	if !strings.Contains(logs.String(), "no config file found") {
 		t.Errorf("no warning logged: %s", logs)
 	}
 	if conf.Logging.Level != "warn" {
@@ -116,7 +116,7 @@ func TestInitKeepsCommandLineLevel(t *testing.T) {
 				t.Errorf("Logging.Level = %q; want %s kept", conf.Logging.Level, level)
 			}
 			// At debug, the environment and loaded config are logged.
-			gotDebug := strings.Contains(logs.String(), "Loaded config:")
+			gotDebug := strings.Contains(logs.String(), "loaded config value")
 			if gotDebug != (level == "debug") {
 				t.Errorf("debug output logged = %v at level %s: %s", gotDebug, level, logs)
 			}
@@ -241,9 +241,9 @@ func TestPrintLoadedConfig(t *testing.T) {
 
 	out := logs.String()
 	for _, want := range []string{
-		`logging.level = \"info\" <string>`,
-		`server.websocket.connection_limit.enabled = true <bool>`,
-		`database.file = \"/tmp/js.db\" <string>`,
+		`key=logging.level value=info type=string`,
+		`key=server.websocket.connection_limit.enabled value=true type=bool`,
+		`key=database.file value=/tmp/js.db type=string`,
 	} {
 		if !strings.Contains(out, want) {
 			t.Errorf("missing %q in:\n%s", want, out)