Projects excluded from static graph constraint checks should not appear in the caches

[cdmihai]

#4379 enables projects to skip isolation constraints on certain references. These references should not appear in the serialized caches when -outputResultsCache is used. If the projects appear in the serialized caches, it can introduce a side channel through undeclared dependencies leak, leading to harder to understand, unstable builds.

[cdmihai]

This turns out to be a bigger problem than expected. If multiple nodes in the graph exempt the same project file, the build results of the exempt project will trickle up and conflict on the first parent that tries to merge them with:

Unhandled Exception: Microsoft.Build.Shared.InternalErrorException: MSB0001: Internal MSBuild Error: Input caches should not contain entries for the same configuration
   at Microsoft.Build.Shared.ErrorUtilities.ThrowInternalError(String message, Exception innerException, Object[] args)
   at Microsoft.Build.Execution.CacheAggregator.InsertCaches(IConfigCache configCache, IResultsCache resultsCache)
   at Microsoft.Build.Execution.CacheAggregator.Aggregate()
   at Microsoft.Build.Execution.BuildManager.ReuseOldCaches(String[] inputCacheFiles)
   at Microsoft.Build.Execution.BuildManager.<BeginBuild>g__InitializeCaches|53_1()
   at Microsoft.Build.Execution.BuildManager.BeginBuild(BuildParameters parameters)

[cdmihai]

The first implementation for this fix, which was the easiest, was to just let exempt projects appear in results file caches. This had a high number of weird side effects, fixed in PR #5222. But the fixes were quite complicated, and after review with @rainersigwald we decided to explore another path, which is to not allow isolation exempt references to be stored in output results file caches, period. But that has consequences of its own which need to be explored.