Add kpm packages command for feed management

[ChengTian]

Had a discussion with @lodejard and @davidfowl .

My work should be based on branch https://github.com/aspnet/KRuntime/tree/feature-feeds-new

1. Remove configuration component from build output path.
2. Delete Blob-related kpm feed operations. Keep the abstraction for future extension.
3. Add kpm build --out-packages <feed_path>, which builds a whole solution and generate a local package feed in <feed_path>.

[ChengTian]

Looks like the functionality of kpm build --out-packages should be put to an individual command, instead of an option.

@lodejard , do you have any suggestion of the name of this new command?
@davidfowl doesn't like kpm build-feed.

My proposal is kpm feed build because it naturally falls into the category of kpm feed. You can do kpm feed build, followed by a kpm feed push, and later kpm feed pull.

[ChengTian]

Talked to @lodejard . This topic needs more efforts to make sure consistent experience. We might want to move this out of beta2 milestone for further discussion.

[davidfowl]

We decided to implement a new command decoupled from build:

kpm feed add [nupkg] [source] - Adds a nuget package to the specified feed
kpm feed push [remote] [source]
kpm feed pull [remote] [source]

The workflow will be:

c:\HelloWorld> kpm build
c:\HelloWorld> kpm feed add bin\Debug\HelloWorld.nupkg %userprofile%\.kpm\packages

We may have a default which means add to the default kpm cache (%userprofile.kpm\packages)

/cc @lodejard

[lodejard]

I think we also talked about it being kpm packages add | push | pull