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/print_test.go

@@ -108,15 +108,15 @@ func TestPrintLog(t *testing.T) {
 				t.Errorf("PrintLog(%v) line %q; want %s", tt.at, l, tt.want)
 			}
 		}
-		if !strings.Contains(logs.String(), "logging.level = warn") {
+		if !strings.Contains(logs.String(), "key=logging.level value=warn") {
 			t.Errorf("PrintLog(%v) missing logging.level:\n%s", tt.at, logs)
 		}
-		// Each line is msg="\t<key> = <value>".
+		// Each line is msg="config value" key=<key> value=<value>.
 		flat := conf.Flatten()
 		var keys []string
 		for _, l := range lines {
-			_, msg, _ := strings.Cut(l, `msg="\t`)
-			key, _, _ := strings.Cut(msg, " = ")
+			_, rest, _ := strings.Cut(l, ` key=`)
+			key, _, _ := strings.Cut(rest, " ")
 			if _, ok := flat[key]; !ok {
 				t.Errorf("PrintLog(%v) line %q: no key", tt.at, l)
 			}