Code completion does not work in REPL for opaque types

[anatoliykmetyuk]

This is becuase opaque type T = Int is desugared to type T = T.T, and autocompletion targets T.T instead of Int. The reason is that the code meant for autocompletion is inferred based on the position of the cursor in the user input. This position is oblivious to the desugaring and other transformation done to the tree as part of the typecheck for code completion. See #6414 for some discussion.