Support legacy value of MSBuildCopyContentTransitively

[stan-sz]

Based on dotnet/msbuild#4865 and

MSBuildPrediction/src/BuildPrediction/Predictors/GetCopyToOutputDirectoryItemsGraphPredictor.cs

Line 42 in 0099629

bool copyContentTransitively = projectInstance.GetPropertyValue(MSBuildCopyContentTransitivelyPropertyName).Equals("true", StringComparison.OrdinalIgnoreCase);

the legacy MSBuildCopyContentTransitively!={true|false} is not supported correctly by the predictor.

[dfederm]

"Unset" behavior differs based on scenario so this may not be solvable.

In MSBuild command-line, unset behaves like true.
In VS/QuickBuild, unset behaves like false.