Init Win7PlusCoreSystem
Convert to byte[] before creating the image reference for project refs
Skip the check for the things already marked as framework references.
- This will make the "not having VS installed" work the default, implicit
references. But it won't work generally. This fix is good enough since
new code will require the frameworkAssemblies section.

#669
Don't make the default hidden dependencies implicit
Fixup nuspec files on restore if the target nuspec doesn't match case

#664
Remove nuspec file from sln and added global.json
Updating comment
Disable crossgen tests for now.
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