Separate pack exclusion from compile exclusion

parent #469 

Now in `project.json`, we have `code-exclude` pattern and `pack-exclude` pattern. They have the same default values:
```c#
new[] { @"obj\**\*", @"bin\**\*", @"**.csproj", @"**.kproj", @"**.user", @"**.vspscc", @"**.vssscc", @"**.pubxml" }; 
```