Fix error when removing file from GameRoot

[HebaruSan]

Problem

If you install a mod that installs a file to GameRoot (e.g. AdvancedFlyByWire), and then remove it, the removal fails:

About to remove:

 * Advanced Fly-By-Wire (Windows) 1.8.3.2
 * Toolbar Controller 1:0.1.9.4
 * ClickThrough Blocker 1:0.1.10.14
 * Zero MiniAVC 1:1.1.0.1
Removing AdvancedFlyByWire...
/ is already absolute
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

In #3180 we fixed KSPPathUtils.NormalizePath to no longer incorrectly convert "/" to "", because those are two different directories, and normalization is supposed to generate a standardized format of the same directory.

That change indirectly confused this loop:

CKAN/Core/ModuleInstaller.cs

Lines 900 to 908 in a494e1d

	// remove the system paths, leaving the path under the instance directory
	var relativeHead = KSPPathUtils.ToRelative(dir, gameDir);
	var pathArray = relativeHead.Split('/');
	var builtPath = string.Empty;
	foreach (var path in pathArray)
	{
	builtPath += path + '/';
	results.Add(KSPPathUtils.ToAbsolute(builtPath, gameDir));
	}

For a file in GameRoot, the absolute path to the game folder is included in the directories parameter of ModuleInstaller.AddParentDirectories, which results in relativeHead containing the empty string. Then in the first iteration of the loop, buildPath is set to /, and ToAbsolute calls NormalizePath on it, after which it is still /, so ToAbsolute throws because that path is already absolute.

Considered and not done

We could change ToAbsolute to use some method other than NormalizePath to massage its inputs. I think it would be better to continue to uncover places in Core where paths are treated sloppily and fix them. ToAbsolute("/", "/myroot/") should throw an exception if ToAbsolute("/afile", "/myroot/") would.

Changes

Now if relativeHead is empty, that means there are no parent folders to check, so we skip that loop.

Fixes #3195.

[DasSkelett]

Do you think this is worth adding tests for? ModuleInstallerDirTest.cs already has some for AddParentDirectories(). I feel like this is something we could accidentally break again by changing code only related over multiple corners.

[HebaruSan]

Yes, I agree that's a good idea.

[HebaruSan]

OK, added a test for that, as well as for Ships/Script since there's already a framework for it.

[DasSkelett]

Great, let's release our users from the install-but-never-uninstall-again trap 😄