Verify correct settings of PrivateAssets

[tom-englert]

After installing a weaver via NuGet, the PrivateAssets="all" is missing in the PackageReference node, which results in the weavers attribute assembly being listed in the copy-local dependencies.

Setting <DevelopmentDependency>true</DevelopmentDependency> in the weavers NuGet package is not an option, since this will create an unusable <IncludeAssets> entry, so after installing a weaver the PrivateAssets="all" must be set manually.

Since this is likely to be forgotten or overlooked, Fody should emit a warning/error when a weaver with the ShouldCleanReference set to true does not have a corresponding PrivateAssets="all" in the project file.

Only usage via PackageReference in the project file will be supported.

(Fixes Fody/PropertyChanged#567)