de-virtualize the /v3 path, remove go.mod from root

[acln0]

As discussed in the #gofrs Slack channel, perhaps we finally have a resolution for #61.

The plan is to de-virtualize the /v3, remove go.mod from the project root, and add forwarding declarations and type aliases in the /v3 subdirectory, followed by cutting v3.2.0.

If anyone is willing to do this work and submit it for review, that would be great. Please ping me when you do. I had previously sent c6f696b which is the other way around. There, the forwarding package is the root package, while /v3 contains the actual code. Perhaps it would be good to base the work on that commit, but we want to do it the other way around. The root package should be the canonical one, and we bite the bullet for a while to support existing modules users using the /v3 subdirectory.

What happens with a potential v4.0.0 in the future remains to be seen.

[itsjamie]

I'll do this in the next four hours.

[theckman]

Based on learnings from @thepudds we've realized this wasn't needed. We instead removed the go.mod and are relying on the reproducible build nature of Go Modules to have it continue to work for those who have depended on the modules version. We are encouraging people move back to the incompatible module by removing v3 from the import string.