Graph cross targeting take two

[cdmihai]

Reimplementation of #4143 to address its issues by adding explicit nodes for inner builds instead of trying to hide their complexity away. It does so by imposing a contract on what crosstargeting means, and requiring the sdk to be self-descriptive about its crosstargeting structure (the property name of the inner build (e.g. TargetFramework), and the property name of the inner build values (e.g. TargetFrameworks)).

• no more races on inner builds. They get deduped in the graph.
• inner builds can have non overlapping references
• inner builds can have arbitrary incoming edges
• clear path forward on how to filter the graph per TargetFramework. Right now if project A depends on crosstargeting project B, all of Bs innerbuilds will be added as speculative edges of A. This is no different from building a solution today, which builds all TFs. If one wants to filter, then they have to flow the desired TF through the graph (using Nuget's GetNearestTF) and clip out the unused speculative edges.

Future PRs:

• update documentation
• tag nodes and edges of interest with extra metadata: outer builds, inner builds, speculative edges
• refactorings, better inner build detection to avoid overbuilding some targets

[cdmihai]

cc @Erikma @dannyvv @yash256

[AndyGerlicher]

Looks good! Only think I see is the ToDot which the more I think about it the more I think it should go.

[rainersigwald]

Some initial thoughts. Haven't looked at the hard parts (the C# code changes) yet.

[rainersigwald]

Reviewed tests. Several questions about graphs that did not match my expectations. Perhaps the reasons will emerge as I look at the implementation (next up).

[rainersigwald]

Can you add some negative tests? Like "doesn't provide the correct metadata to understand the inner build" -- what's the failure look like? InvalidProjectFileException?

[rainersigwald]

Ok, my review is complete. Many comments, but the big conceptual thing I have is about the edge elision/promotion around non-graph-root outer builds.

[rainersigwald]

LGTM pending filing the overbuild bug.