"kpm pack" should also pack source files outside of project root dir

[ChengTian]

To reproduce the problem, do kpm pack to pack Microsoft.Framework.PackageManager project in KRuntime and then run the packed output.

Note that in the project.json of Microsoft.Framework.PackageManager we have

"code": "**\\*.cs;..\\Microsoft.Framework.ApplicationHost\\Impl\\**\\*.cs"

It means this project is referencing some source files outside of project dir. We should kpm pack the externally referenced source files, too.