Combine AbstractPos, PosAdapter, and Pos

Also move `SourcePath` into `libutil`.

These changes allow `error.hh` and `error.cc` to access source path and
position information, which we can use to produce better error messages
(for example, we could consider omitting filenames when two or more
consecutive stack frames originate from the same file).

src/libutil/input-accessor.hh

@@ -0,0 +1,27 @@
+#pragma once
+///@file
+
+#include "source-accessor.hh"
+#include "ref.hh"
+#include "repair-flag.hh"
+
+namespace nix {
+
+MakeError(RestrictedPathError, Error);
+
+struct InputAccessor : virtual SourceAccessor, std::enable_shared_from_this<InputAccessor>
+{
+    std::optional<std::string> fingerprint;
+
+    /**
+     * Return the maximum last-modified time of the files in this
+     * tree, if available.
+     */
+    virtual std::optional<time_t> getLastModified()
+    {
+        return std::nullopt;
+    }
+
+};
+
+}