Make rootFS's showPath() render the paths from the original accessors

This makes paths in error messages behave similar to lazy-trees,
e.g. instead of store paths like

       error: attribute 'foobar' missing
       at /nix/store/ddzfiipzqlrh3gnprmqbadnsnrxsmc9i-source/machine/configuration.nix:209:7:
          208|
          209|       pkgs.foobar
             |       ^
          210|     ];

you now get

       error: attribute 'foobar' missing
       at /home/eelco/Misc/eelco-configurations/machine/configuration.nix:209:7:
          208|
          209|       pkgs.foobar
             |       ^
          210|     ];

src/libutil/forwarding-source-accessor.hh

@@ -0,0 +1,57 @@
+#pragma once
+
+#include "source-accessor.hh"
+
+namespace nix {
+
+/**
+ * A source accessor that just forwards every operation to another
+ * accessor. This is not useful in itself but can be used as a
+ * superclass for accessors that do change some operations.
+ */
+struct ForwardingSourceAccessor : SourceAccessor
+{
+    ref<SourceAccessor> next;
+
+    ForwardingSourceAccessor(ref<SourceAccessor> next)
+        : next(next)
+    {
+    }
+
+    std::string readFile(const CanonPath & path) override
+    {
+        return next->readFile(path);
+    }
+
+    void readFile(const CanonPath & path, Sink & sink, std::function<void(uint64_t)> sizeCallback) override
+    {
+        next->readFile(path, sink, sizeCallback);
+    }
+
+    std::optional<Stat> maybeLstat(const CanonPath & path) override
+    {
+        return next->maybeLstat(path);
+    }
+
+    DirEntries readDirectory(const CanonPath & path) override
+    {
+        return next->readDirectory(path);
+    }
+
+    std::string readLink(const CanonPath & path) override
+    {
+        return next->readLink(path);
+    }
+
+    std::string showPath(const CanonPath & path) override
+    {
+        return next->showPath(path);
+    }
+
+    std::optional<std::filesystem::path> getPhysicalPath(const CanonPath & path) override
+    {
+        return next->getPhysicalPath(path);
+    }
+};
+
+}