fix(entry_index): prevent recursive indexing on overrides

[jamestrew]

When indexing into an entry inside of the override function, the same key you are overriding, this causes an infinitely recursive indexing.

eg.

local opts = {
  entry_index = {
    text = function(entry)
      -- accessing the same key you're overriding in the override
      -- function will lead to stack overflow

      return "foobar " .. entry.text -- bad
    end,
  }
}

This change will let the entries keep track of indexes that have been overridden and prevent recursive indexing.