fix: INetworkMessageILPP failing to output registration code for network messages

[ShadauxCat]

There were two issues at play here:

1. We were looking for netstandard.dll with a simple single-layer-deep assembly reference search. In our development environments, apparently our assemblies all have it as a direct reference. In some users' environments, it's not. The search was updated to be recursive.
2. Apparently in some environments, Type and List<> aren't even defined in netstandard.dll anyway... so I reverted those things back to importing using typeof(), which should be safe and ok for system types since they don't have any differences in structure based on conditional compilation... so netstandard.dll isn't even needed anymore, but I kept the change from part 1 because it's "more correct" anyway and will prevent us from potentially hitting any future edge cases involving assemblies that have indirect references to assemblies we need to import.

Testing and Documentation

• No tests have been added.
• No documentation changes or additions were necessary.

[ShadauxCat]

Found out what was different about other users' environments - affected users were using .NET Framework instead of .NET Standard 2.1 in their player settings. That affects the locations of system types. Importing system types using typeof() instead of trying to look for them manually makes us immune to that variance.

[NoelStephensUnity]

LGTM!
💯