Add buildTransitive to fix netcoreapp3 on windows vs not

[clairernovotny]

Due to the way netcoreapp3 works, after discussion with @terrajobst, we need to use the new buildTransitive folder with NuGet to supply two versions of the library for netcoreapp3: 1 with the overloads and one without.

https://github.com/NuGet/Home/wiki/Allow-package--authors-to-define-build-assets-transitive-behavior

We can switch them on a condition (UseWPF/UseWindowsForms/something else?) from a targets file for that TFM and be sure the targets will be applied across all consumers.

Plan:

• Build netcoreapp3 twice, with and without
• put a blank entry in \lib\netcoreapp3.0 (_._).
• Use a targets file in buildTransitive\netcoreapp3.0 to include an ItemGroup with a Reference element. Will conditionally use the right version of the library based on the properties.

[glennawatson]

Be interested to see how you you approach this, will likely emulate for ReactiveUI.

[clairernovotny]

This is fixed in #859