Mount flake input source accessors on top of storeFS

This way, we don't need the PathDisplaySourceAccessor source accessor
hack, since error messages are produced directly by the original
source accessor.

In fact, we don't even need to copy the inputs to the store at all, so
this gets us very close to lazy trees. We just need to know the store
path so that requires hashing the entire input, which isn't lazy. But
the next step will be to use a virtual store path that gets rewritten
to the actual store path only when needed.

src/libutil/source-accessor.hh

@@ -118,7 +118,7 @@ struct SourceAccessor : std::enable_shared_from_this<SourceAccessor>
         std::string typeString();
     };
 
-    Stat lstat(const CanonPath & path);
+    virtual Stat lstat(const CanonPath & path);
 
     virtual std::optional<Stat> maybeLstat(const CanonPath & path) = 0;
 
@@ -214,8 +214,6 @@ ref<SourceAccessor> getFSSourceAccessor();
  */
 ref<SourceAccessor> makeFSSourceAccessor(std::filesystem::path root);
 
-ref<SourceAccessor> makeMountedSourceAccessor(std::map<CanonPath, ref<SourceAccessor>> mounts);
-
 /**
  * Construct an accessor that presents a "union" view of a vector of
  * underlying accessors. Earlier accessors take precedence over later.