Restrict readFile context to references that appear in the string

When calling `builtins.readFile` on a store path, the references of that
path are currently added to the resulting string's context.

This change makes those references the *possible* context of the string,
but filters them to keep only the references whose hash actually appears
in the string, similarly to what is done for determining the runtime
references of a path.

src/libstore/references.hh

@@ -27,6 +27,19 @@ public:
     void operator () (std::string_view data) override;
 };
 
+class PathRefScanSink : public RefScanSink
+{
+    std::map<std::string, StorePath> backMap;
+
+    PathRefScanSink(StringSet && hashes, std::map<std::string, StorePath> && backMap);
+
+public:
+
+    static PathRefScanSink fromPaths(const StorePathSet & refs);
+
+    StorePathSet getResultPaths();
+};
+
 struct RewritingSink : Sink
 {
     std::string from, to, prev;