reject packages with filenames that are case-insensitive equal

[jonasfj]

Such packages cannot be extracted on windows. as it has a case-insensitive (case-preserving) file system.

Example: dart-lang/pub#2072

We might also have to scan all existing packages for this... I'm not sure we can remove them... that might be something we can editorial decision on... I suspect all cases of this is mistakes :)