[SBRP] Build/BuildTransitive/BuildMultiTargeting files should be included in SBRP packages

[ViktorHofer]

Some of our nuget packages add msbuild files to the /build, /buildTransitive or /buildMultiTargeting folder in nuget packages. Those often add metadata to the compilation that is currently missed when using SBRP packages which creates an artificial difference with the baseline.

We should consider including those files and also not filtering them out if they are placed in excluded TFM folders as they apply to >= that TFM.