Tooling support for props and targets files outside of the project file

Currently the tooling around project properties (eg. Output Type, Assembly info etc.) and items (eg. Nuget references) supports reading from a number of places, such as the project file itself, but also targets imported using the element in the project file, or implicit imports that are based on folder structure, by naming files Directory.Build.targets or similar. Unfortunately the project system only supports writing to the project file itself, so for projects with these more advanced configurations they either need to not use the tooling at all in some scenarios (eg, not use Nuget Package Manager), or must deal with fixing up the changes the tooling makes by moving things to the right files.

Most of the projects in the https://github.com/dotnet/ org, including this one, are using a pattern of centralising Nuget packages and versions in a single file, and even this relatively simple step is not supported.