We used to do the following:
1. Have abstract task classes in the Xamarin.*.Task.Core assembly (with all
the actual code for the task in question to work properly on macOS).
2. Subclassed task in the Xamarin.*.Task assembly, which did nothing.
3. On Windows we'd inject a different Xamarin.*.Task assembly, with
Windows-specific overrides for the implementation in the abstract base
class.
However, we no longer do point 3, which means that we no longer need to split
our tasks across two assemblies.
This means that we can remove the Xamarin.\*.Task.Core assemblies, and move all
the code into the corresponding Xamarin.\*.Task assembly instead.
This simplifies our code and speeds up the build.
There are more simplifications that can be done; those will come in later PRs.
b9a60c4d7e