Implement dotted identifiers

Like `System.Abs` or `MyProgram.Variable`:

```pascal
Program RedefinitionOfAbsForReals;

Function Abs(X: Real): Real;
Begin
    If X >= 0 Then Abs := X Else Abs := -X;
End;

Var
    X: Real;

Begin
    X := -4.2;
    WriteLn(System.Abs(X), ' ', Abs(X), ' ', RedefinitionOfAbsForReals.Abs(X));
End.
```
