std.getopt: make it an error to have options that differ only by case when case insensitive

[dlangBugzillaToGithub]

dhasenan (@dhasenan) reported this on 2018-10-26T04:32:17Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=19335

Description

void main(string[] args)
{
    import std.getopt;
    import std.stdio;
    int a, A;
    getopt(args, "a", &a, "A", &A);
    writefln("a=%s, A=%s", a, A);
}

Compile, run with `-a 5 -A 10`.

The expected result is an exception saying that option `A|a` is multiply defined, hopefully also mentioning that the case-sensitive flag exists.

The actual result is that `a` is set twice and `A` is not set.

This exception should also trigger when you define an argument named `H`.

(I would also argue that case-insensitive is a terrible default, but that's unlikely to change.)