Selection from a module (`M.x`) as syntactic sugar (`open M in x`) might be counter-intuitive

This syntactic sugar allows to select variables that are not publicly available in the module, but defined in other opened modules. This might lead to very counter-intuitive behavior, when variable is defined as private, as in the following code:
```
module M
let x = 42
end
let x = 13
let y = M.x
```
Current implementation raises no errors and assigns 13 to `y`.

This issue was originally reported by @kwasielewski