load KSP assemblies from the configured KspDir env var

This should make it possible to clone the repo and just hit build. (If there's a KspDir env var already set up)

Without $(KspDir), an error will be shown in the IDE/msbuild that explains which env var to create or where to put the msbuild property.

Old PR, but this can be accomplished with reference paths, which are stored in the uncommitted .csproj.user and thus are not dependent on where you have KSP installed or any environment variables