Catch nested GameData folders in Netkan

[HebaruSan]

Motivation

#2788 created Netkan validation checks based on the legacy Perl bot and the pull request validation scripts. There's one remaining check that isn't migrated yet:

https://github.com/KSP-CKAN/xKAN-meta_testing/blob/73cd8a319ae8833e40b52b49976afe49d00db30b/NetKAN/build.sh#L455-L462

    # Check for Installations that have gone wrong.
    gamedata=($(find dummy_ksp/GameData/. -name GameData -exec sh -c 'if test -d "{}"; then echo "{}";fi' \;))
    if (( ${#gamedata[@]} > 0 ))
    then
      echo "GameData directory found within GameData"
      printf '%s\n' "Path: ${gamedata[@]}"
      exit 1;
    fi

In KSP-CKAN/NetKAN#7587 we discovered that MoreHabPlanets' current latest version was blocked by this check. Presumably it was fine when first indexed, then the format of the ZIP changed later. This could have been caught by the bot as soon as that release was inflated if this check was in Netkan.

Changes

Now the InstallsFilesValidator looks for multiple occurrences of GameData in a mod's files' installation paths and throws an exception if found. This will allow us to adjust metadata to fix this problem before such modules are indexed.

[techman83]

Additional check LGTM (from my vague C# knowledge). I don't see any reason not to merge this as is.