Add flag to init to tweak how revision constraints are imported

[carolynvs]

It's frustrating for dep init to throw away all the constraints, especially for some of the imported configs that don't have constraints at all. dep is opinionated about avoiding pinning, but if someone really wants to, we shouldn't stand in the way. It's especially helpful for more complicated dep migrations, where it's not an immediately switchover.

I'd like to see something like dep init --strict which would do a couple things:

1. Don't throw away revision constraints
2. Don't guess at constraints, faithfully import what is found
3. Return a non-zero exit code and print a warning when the imported lock doesn't match the final lock. (see Note diffs b/w imported and solved lock #1475)

[sttts]

This will help a lot to publish Gopkg.tomls for k8s.io/client-go using the https://github.com/kubernetes/publishing-bot.

[mvdan]

Dep was officially deprecated earlier this year, and the proposal to archive this repository was accepted. As such, I'm closing outstanding issues before archiving the repository. For any further comments, please use the proposal thread on the Go issue tracker. Thanks!