Reduce number of files in packed output

- Nupkgs, nuspecs and SHA files are not copied to output packages folder
- Only copy assemblies needed by the frameworks being packed with
- Use frameworks in project.json if '--runtime' was not specified
- Use values of '--runtime' to filter frameworks in project.json
- Use SHAs from global.json when resolving KRE_PACKAGES_CACHE
- Copy tools dlls from source packages to bin folder of output