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.
Renaming `dst` to `dst1` is just necessary to increase code consistency.