Only delete diversely capitalized directories once on Windows

[HebaruSan]

Problem

1. Reboot into Windows
2. Install LTechContinued 1:0.5.2.2 or earlier
3. Try to uninstall it

Kan een gedeelte van het pad C:\Games\Kerbal Space Program\GameData\LTech\Parts\Science\SkyLab niet vinden.
Error during installation!
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If you suspect a metadata problem: https://github.com/KSP-CKAN/NetKAN/issues/new/choose
If you suspect a bug in the client: https://github.com/KSP-CKAN/CKAN/issues/new/choose

Cause

Up until its latest release (see linuxgurugamer/L-Tech#2), this mod had a Skylab folder and a SkyLab folder right next to each other:

On Windows, these install into a single SkyLab folder, but the Registry remembers the different names.

At uninstall, CKAN tries to remove both directory names. The first succeeds, and the second throws an error because it's already gone.

Changes

Now the HashSet<string> that we use to track folders to delete is case insensitive on Windows, similar to what we did in #3479. This will make sure that we only delete such folders one time.

Fixes #3527.

[DasSkelett]

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