fix(hover): use String.to_atom/1

Erlang modules from dependencies will not have been loaded on the LSP node, so the atoms
of their module names will not have been created yet

lib/next_ls.ex

@@ -317,7 +317,7 @@ defmodule NextLS do
         [reference] ->
           mod =
             if reference.module == String.downcase(reference.module) do
-              String.to_existing_atom(reference.module)
+              String.to_atom(reference.module)
             else
               Module.concat([reference.module])
             end