Save and load game version for fake KSP2 instances

[HebaruSan]

Problems

Two problems came up during KSP-CKAN/KSP2-NetKAN#125:

• The meta tester always fails to install KSP2 mods that aren't compatible with the latest game version
• ckan show --with-versions lists multiple copies of the same version of a mod when the meta tester runs it

Initially I thought the former was caused by how the meta tester's Python code handles game version build numbers, but KSP-CKAN/xKAN-meta_testing#97 didn't fix it.

Causes

• Currently a fake KSP2 instance is always treated as the current game version, because the game version isn't saved in a way that KerbalSpaceProgram2.DetectVersion can retrieve. That function uses FileVersionInfo.GetVersionInfo(path).ProductVersion to examine the game's executable, and we can't generate an executable on the fly. The game version we pass on the command line is saved to a readme.txt file but otherwise ignored.
• With the recent repo refactor, when a mod version exists in multiple repos (which is how the meta tester works), both copies are returned by Registry.AvailableByIdentifier

Changes

• Now when we create a fake KSP2 instance, instead of creating KSP_x64.exe as an empty file, we save the game version in it
• Now if we fail to get the KSP2 game version from FileVersionInfo.GetVersionInfo(path).ProductVersion, we fall back to loading the text of the file as per the above point
• Now Registry.AvailableByIdentifier filters out duplicates