CLI should share build graph information across command invocations

[TheRealPiotrP]

Steps to reproduce

dotnet new
dotnet restore
dotnet build

Expected behavior

A given Project Context is generated once and shared across requests, potentially spanning multiple process invocations

Actual behavior

Even in this simple scenario project contexts are created rampantly, leading to multiple parsings of project.json and project.lock.json

[TheRealPiotrP]

@troydai @anurse @pakrym can we somehow leverage ProjectModelServer to reduce parsing of the graph?

[pakrym]

I don't think we should share it across invocations, but if we move builtin commands to be inproc we can share it with them.

[pakrym]

And we should consider creating project contexts and doing other things slow things like LibraryExporter.GetAllExports() in less places, ideally ones per project.

[livarcocc]

We don't have ProjectModel anymore.