feat: add a replace command that can be used to handle module relocation and rename

[xNok]

I discovered your utility via Renovate, which uses it to handle the major upgrade workflow. The module is conveniently called mod, so it could be extended to auto-resolve all the mod-related issues.

Renovate recently introduced the "replacement" feature that can be used to update the dependencies for which the package name has changed. Unfortunately, the gomod manager is not supported, so I am planning to do the work and get it implemented.

There are a few cases where this happens in go

• Repository changes owners or is given to a different organisation
• Migration to a different repository hosting solution Github <-> Gitlab <-> Bitbucket <-> etc
• Introducing registry or vanity URLs.

My renovate branch for this feature: https://github.com/xNok/renovate/tree/replacement-for-gomod