Unneeded vendor packages in glide.lock are kept

[sgotti]

The glide.lock file may contain unneeded packages. This can happen with unneeded entries in glide.yaml or with not fine grained ones (for example having just github.com/coreos/etcd when only the github.com/coreos/etcd/client pkg is needed will have a glide.lock entry with a lot of etcd packages not needed by the project)

A way to get the real needed pkgs list will be to use the glide list json output. But, since we want to also keep the base package license files, there's the need to also get the base package path. Actually this isn't exported so a solution will be to recycle the glide functions that do this, but this is suboptimal.

[sgotti]

Probably we could just use the glide list output to get the needed packages and the glide.lock to retrieve the base package path.

[sgotti]

Resolved by #21