Introduce FSInputAccessor and use it

Backported from the lazy-trees branch. Note that this doesn't yet use
the access control features of FSInputAccessor.

src/libexpr/paths.cc

@@ -1,10 +1,11 @@
 #include "eval.hh"
+#include "fs-input-accessor.hh"
 
 namespace nix {
 
 SourcePath EvalState::rootPath(CanonPath path)
 {
-    return path;
+    return {rootFS, std::move(path)};
 }
 
 }