Preliminary option to not pass assembly reference to IKVM.Exporter

[AliveDevil]

In iterate-ch/cyberduck we are passing assemblies to IKVM.NET.Sdk that were previously converted by IKVM.NET.Sdk.
B(IKVM.NET.Sdk) depends on A(IKVM.NET.Sdk), which was built by Maven, so B doesn’t know there is in fact a hidden ProjectReference - just not MSBuild-style.

In a recent change the reexport of IKVM-assemblies was turned into an error, thus failing the build.

Workaround at the moment is to set <JavaClasspath>\\?\</JavaClasspath> so the MSBuild targets don’t consume these reference in the _ExportReferences-stage, but are referenceable in the CoreCompile-stage.

Proposal is to preliminarily implement a _HideFromJavaLang reference item metadata, so that the workaround is not needed (unknown consequences in the future).
This name is temporary and underscored to mark its dangerous and temporary nature.