Move to go modules

[chenrui333]

Issue tracker, containerd/containerd#3031

[lumjjb]

@chenrui333 thanks for the PR! Looks good but CI is failing. I believe may be because of this line:

https://github.com/containerd/imgcrypt/blob/master/script/setup/install-runc#L26

It looks like go get behavior is a bit different in 1.13. We can update this to just manually git clone and build runc instead. That should work for our purposes. This will be backward compatible with 1.12 as well. (we can also have both 1.12 and 1.13, that should be fine as well.

vendor.conf looks like it should be removed and this line should reference go.mod instead as well.

https://github.com/containerd/imgcrypt/blob/master/script/setup/install-runc#L24

Which needs to be updated to parse go mod instead of vendor.conf.

[chenrui333]

@lumjjb I will spend some time tomorrow on this, too busy today. :)

[lumjjb]

Awesome! No worries. Thanks! :D

[chenrui333]

FYI, I will spend more time on this later today or tomorrow. :)

[lumjjb]

@chenrui333 Happy New Year! How are things going?

[chenrui333]

@lumjjb Happy new year!!

[lumjjb]

LGTM! Thanks for the contribution!