Factor out nix::maybeLstat

This function is nice for more than `PosixSourceAccessor`. We can make a
few things simpler with it.

Note that the error logic slightly changes in some of the call sites, in
that we also count `ENOTDIR` and not just `ENOENT` as not having the
file, but that should be fine.

src/libutil/file-system.hh

@@ -84,6 +84,7 @@ bool isDirOrInDir(std::string_view path, std::string_view dir);
  */
 struct stat stat(const Path & path);
 struct stat lstat(const Path & path);
+std::optional<struct stat> maybeLstat(const Path & path);
 
 /**
  * @return true iff the given path exists.