add vendor

[xiekeyang]

Signed-off-by: xiekeyang [email protected]

[wking]

Instead of adding these under tools/vendor, I'd rather have them under tools/cmd/vendor. That way the library packages will not use locally-vendored code (which can lead to problems, see opencontainers/image-spec#528 and especially this comment).

It would be nice to have the Makefile's Go-testing targets temporarily link tools/cmd/vendor to tools/vendor for reliable tests, but that can happen as a second phase.

The packages we depend on also seem fairly stable, so I'm also ok if we want to punt on vendoring for now and only add it if/when we get more build/install tooling around our installed command. When we land vendoring, it would be good to also land docs on how we expect the vendor directory to be maintained. I'll expect we want to use some tool like Godep, Glide, etc.

[xiekeyang]

So I move vendor to tools/cmd dir, and later add link in Makefile. But I honestly don't like glide a little. I think manually updating the vendor very occasionally is OK.

[xiekeyang]

LGTM