move implementation to internal/ directory (#828)

internal/daemon/logging/trace/trace_debug.go

@@ -0,0 +1,19 @@
+package trace
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/zrepl/zrepl/internal/util/envconst"
+)
+
+const debugEnabledEnvVar = "ZREPL_TRACE_DEBUG_ENABLED"
+
+var debugEnabled = envconst.Bool(debugEnabledEnvVar, false)
+
+func debug(format string, args ...interface{}) {
+	if !debugEnabled {
+		return
+	}
+	fmt.Fprintf(os.Stderr, format+"\n", args...)
+}