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/logging/logging_test.go

@@ -18,9 +18,11 @@ func saveGlobals(t *testing.T) {
 	t.Helper()
 	level := logLevel.Level()
 	def := slog.Default()
+	out, detect := terminal, isTerminal
 	t.Cleanup(func() {
 		logLevel.Set(level)
 		slog.SetDefault(def)
+		terminal, isTerminal = out, detect
 		Shutdown()
 	})
 }
@@ -155,3 +157,52 @@ func TestLogToTerminal(t *testing.T) {
 		t.Fatal("terminal handler ignores the package level")
 	}
 }
+
+// On an interactive terminal the handler, and loggers derived from it, report
+// that they write to the terminal; when stdout is a file or pipe they don't.
+func TestWritesToTerminal(t *testing.T) {
+	for _, interactive := range []bool{true, false} {
+		saveGlobals(t)
+		isTerminal = func(*os.File) bool {
+			return interactive
+		}
+
+		LogToTerminal()
+
+		derived := slog.Default().With("component", "test").WithGroup("g")
+		for name, h := range map[string]slog.Handler{
+			"default": slog.Default().Handler(),
+			"derived": derived.Handler(),
+		} {
+			if got := WritesToTerminal(h); got != interactive {
+				t.Errorf(
+					"interactive %v, %s: WritesToTerminal = %v",
+					interactive, name, got,
+				)
+			}
+		}
+	}
+	if WritesToTerminal(slog.DiscardHandler) {
+		t.Error("another handler reported as the terminal")
+	}
+}
+
+func TestPrintToTerminal(t *testing.T) {
+	saveGlobals(t)
+	out, err := os.Create(filepath.Join(t.TempDir(), "terminal"))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer out.Close()
+	terminal = out
+
+	PrintToTerminal("line one\nline two")
+
+	got, err := os.ReadFile(out.Name())
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != "line one\nline two\n" {
+		t.Errorf("printed %q; want the text as it is, then a newline", got)
+	}
+}