Support one more targets file which will be evaluated immediately after project file but before targets files from NuGet packages.

I want to define properties in `Directroy.Build.props` files and let projects to opt in or out of some behavior.
And then I want to do some actions like referencing projects, changing assembly name, etc.
The problem is that currently I can do it either with `Directory.Build.targets` file or by explicitly importing the custom `targets` file.

If I make changes in `Directory.Build.targets` file I mess with the behavior of NuGet packages.
See [this issue](https://github.com/microsoft/WindowsAppSDK/issues/4606) as an example.
I have similar problem with MSTest engine as well, when I change AssemblyName in `targets` file.

So, I'd like to have the convenience of automatic import and early evaluation.