Rename local variables to avoid conflict with local imports

This is currently blocking my PR https://github.com/D-Programming-Language/dmd/pull/3407

A reduced test case to explain what happened.

```d
void main() {
    int std = 1;
    import std.uni;

    std = 2;    // OK
    auto s = std.uni.toLower('A');  // NG
}
```

Currently local variable name `std` is preferred and fully qualified name `std.uni` is implicitly masked. But https://github.com/D-Programming-Language/dmd/pull/3407 changes the name `std` ambiguous in function scope.

I'm not sure what should be the language behavior, but at least renaming local variables would be better to reduce potential of bug.
In this unittest block, renaming is not necessary. But I'd like to increase code consistency.