Avoid copying dependencies to output directory of a desktop class library

Currently dependencies of a class library that targets netXX are copied to the output directory. This wastes a lot of disk space. Libraries targeting netstandard do not have this problem.

The issue names no files, tests, or entry points. Start by reproducing the output of a desktop class library targeting netXX and compare it with a netstandard library; trace the dependency-copying build target. Done means desktop class library dependencies are not unnecessarily copied while the existing netstandard behavior remains unchanged.

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