[Issue 21227] import(".\\file") doesn't work on Windows

[d-bugmail at puremagic.com]

https://issues.dlang.org/show_bug.cgi?id=21227

--- Comment #3 from Andrey Zherikov <andrey.zherikov at gmail.com> ---
I'm not an expert in this subject but IMO the restrictions should simple:
 - Do not allow absolute path. There is FileName.absolute function that can be
used: https://github.com/dlang/dmd/blob/master/src/dmd/root/filename.d#L103
 - Do not allow reference to parent (i.e. ".." ) directory anywhere in the path

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