changes to kpm pack to not include files that are not needed during runtime

[lodejard]

To minimize upload size and file-server writes, kpm pack should avoid putting files into the output location if it's certain they're not needed at runime.

Assuming the X.nuspec and X.VER.nupkg.sha files are the only markers needed at runtime - don't copy the X.VER.nupkg to the output location.

Given the target runtimes which the application is designated to run on (inside project.json being packed - the values aspnet50 and/or aspnetcore50) only copy the X/lib/*/*.dll files which are the best match for those target runtimes.

Do not bother expanding .xml doc comment files in lib folders. (?? should pdb files be uploaded, or do not bother sending those ??)

Further refinement on that - if kpm pack is given --runtime switch to include a copies of the KRE - ignore project.json runtimes which fall outside of those switches. (if a project.json has both aspnet50 and aspnetcore50 targets, and the pack --runtime switch is core coreclr, behave as if the project.json only contained the aspnetcore50 target)

So ideally - for a package dependency X - the packed files should be X.nuspec, X.VER.nupkg.sha, and at most one lib/ZZZ/X.dll per target runtime

[lodejard]

Even further refinement would be, when including --runtime KRE files, for kpm pack to reduce even further what is needed in those directories. But that should probably be files as a different issue.