move implementation to internal/ directory (#828)

internal/zfs/zfs_debug.go

@@ -0,0 +1,21 @@
+package zfs
+
+import (
+	"fmt"
+	"os"
+)
+
+var debugEnabled bool = false
+
+func init() {
+	if os.Getenv("ZREPL_ZFS_DEBUG") != "" {
+		debugEnabled = true
+	}
+}
+
+//nolint:deadcode,unused
+func debug(format string, args ...interface{}) {
+	if debugEnabled {
+		fmt.Fprintf(os.Stderr, "zfs: %s\n", fmt.Sprintf(format, args...))
+	}
+}