Migrate to GOVENDOREXPERIMENT

[akavel]

Proposed approach:

1. Switch the code to use vendor/ subdir instead of _vendor/src;
2. Write tests verifying it works with go with empty GOPATH;
3. Review & merge;
4. Move the stuff in our internal codebase to vendor/, adjust relevant scripts to use GOVENDOREXPERIMENT=1 everywhere; try living with that for a while;
5. If we're ok with it, go on and close this issue; if not, make it a command-line option (vendor/ by default, but can use _vendor/src/ instead). [Emergency backup plan: pin the older version of vendo in our internal codebase's _vendor/.]

Important issue to research and resolve later: in general, dependencies may have their own vendor/ subdirs; verify how vendo behaves if they do. We don't want to duplicate the packages in the root vendor/. Do the stdlib's go/build packages solve that for us in Go 1.5 or 1.6 automatically? (see: golang/go@0c428a5)