Move ParseSink to its own header

We will soon add a new implemenation so the one for NARs in `archive.cc`
isn't the only one.

src/libutil/archive.hh

@@ -3,6 +3,7 @@
 
 #include "types.hh"
 #include "serialise.hh"
+#include "fs-sink.hh"
 
 
 namespace nix {
@@ -72,22 +73,6 @@ time_t dumpPathAndGetMtime(const Path & path, Sink & sink,
  */
 void dumpString(std::string_view s, Sink & sink);
 
-/**
- * \todo Fix this API, it sucks.
- */
-struct ParseSink
-{
-    virtual void createDirectory(const Path & path) { };
-
-    virtual void createRegularFile(const Path & path) { };
-    virtual void closeRegularFile() { };
-    virtual void isExecutable() { };
-    virtual void preallocateContents(uint64_t size) { };
-    virtual void receiveContents(std::string_view data) { };
-
-    virtual void createSymlink(const Path & path, const std::string & target) { };
-};
-
 /**
  * If the NAR archive contains a single file at top-level, then save
  * the contents of the file to `s`.  Otherwise barf.

src/libutil/fs-sink.hh

@@ -0,0 +1,25 @@
+#pragma once
+///@file
+
+#include "types.hh"
+#include "serialise.hh"
+
+namespace nix {
+
+/**
+ * \todo Fix this API, it sucks.
+ */
+struct ParseSink
+{
+    virtual void createDirectory(const Path & path) { };
+
+    virtual void createRegularFile(const Path & path) { };
+    virtual void closeRegularFile() { };
+    virtual void isExecutable() { };
+    virtual void preallocateContents(uint64_t size) { };
+    virtual void receiveContents(std::string_view data) { };
+
+    virtual void createSymlink(const Path & path, const std::string & target) { };
+};
+
+}