Support SkipNonexistentTargets in project reference target protocol

[DmitriyShepelev]

Fixes #4252

Context

This PR extracts the relevant logic from the closed #5297, which adds support for the SkipNonexistentTargets metadatum on the ProjectReferenceTargets item:

<ProjectReferenceTargets Include='<some-target>' Targets='<some-target1>;<some-target2>' SkipNonexistentTargets='<boolean>'>

If SkipNonexistentTargets is true, then any targets in Targets are skipped if they're nonexistent. SkipNonexistentTargets cannot be added to ProjectReferenceTargets items whose Targets contain .default or .projectReferenceTargetsOrDefaultTargets, which represent the default targets and targets specified on the ProjectReference item (with fallback to default targets if none are specified), respectively.

Changes Made

• GetTargetLists filters out skippable nonexistent targets on referenced projects.
• Determining whether a target is skippable required storing the defined project targets in BuildResults to ensure that corresponding BuildRequestConfigurations on the build manager node have set project targets if the build manager node created a configuration based on a request from an external node but hadn't received a result (since the project may not have been loaded locally and thus the project targets would be unknown).
• Added SkipNonexistentTargets='true' to GetTargetFrameworks since in the non-graph case it is added to the relevant MSBuild task.

Testing

UTs and manual testing with /graph and /graph /isolate (the latter run without restore being called due to #6856) on the erroring repos I saw when testing #8249.

Notes

Addressing this since it came up when testing #8249.

cc @dfederm

[dfederm]

Is there a specific use-case for this? I see a theoretical one, but I'm not aware of anything in the typical p2p protocol which uses SkipNonexistentTargets

[DmitriyShepelev]

Is there a specific use-case for this? I see a theoretical one, but I'm not aware of anything in the typical p2p protocol which uses SkipNonexistentTargets

.wixproj projects don't define GetTargetFrameworks nor GetTargetFrameworksWithPlatformForSingleTargetFramework so, if they happen to be referenced, MSBuild will emit errors about nonexistent targets being called.

[rokonec]

/azp run

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[DmitriyShepelev]

/azp run

[azure-pipelines]

Commenter does not have sufficient privileges for PR 8330 in repo dotnet/msbuild

[rokonec]

/azp run

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[JaynieBai]

@DmitriyShepelev Please help resolve the conflicts