"kpm pack" should consider wrapped projects as special cases

[ChengTian]

@davidfowl found a bug when we kpm pack an app which references results of kpm wrap. Reproduce steps:

1. kpm wrap a csproj
2. Reference the wrapped csproj from a kproj
3. kpm pack the kproj
4. Run the packed app

The packed app cannot run because kpm pack only copies source code and it doesn't consider assembly & pdb in project.json of dependency projects.

Here is the solution @davidfowl gives:
kpm pack --no-source can resolve this problem. We can automatically "no source" the wrapped projects. That is, if you declare assembly and pdb, we treat the project like a package and build package out of it during kpm pack.