std.file: Long paths support on Windows requires explicit \\?\ prefix

[dlangBugzillaToGithub]

kinke reported this on 2021-06-21T15:28:51Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=22059

Description

```
void main()
{
    import std.file;
    // works
    assert(exists(`\\?\C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`));
    // fails
    assert(exists(`C:\Users\Martin\AppData\Local\dub\packages\mir-algorithm-1.0.1\mir-algorithm\.dub\build\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D\library-unittest-windows-x86_64-ldc_2079-0C3BC09825103C01E6AE45507110137D.obj`));
}
```

The prefix shouldn't be required and added automatically, as the Visual Studio 2019 linker doesn't require that prefix, nor does the LLD linker, nor does DMD, ...

Phobos should essentially use something very similar to DMD's extendedPathThen() helper: https://github.com/dlang/dmd/blob/ef39b070819216a8d9835e2a66d3e365420763ec/src/dmd/root/filename.d#L1146