Use ILRepack due to conflicts

[bradwilson]

To prevent conflicts with xunit.runner.utility and xunit.runner.reporters, we should ILMerge everything (except xunit.abstractions).

We've already done this for the runners in v3: https://github.com/xunit/xunit/blob/3c0ff591e4a33f79df3372a997cadeb5af3c733a/src/Directory.Build.targets#L37-L84

See: xunit/xunit#2771 (reply in thread)