all: Become a Go module (fixes #5148)

[calmh]

Purpose

Use the new hotness. Build cleanly outside GOPATH. Use modern tools for vendoring.

The changes in more detail:

• Our dependencies are now declared in go.mod & go.sum. Managing these is done using go get, go get -u, go mod tidy, go mod vendor while in the modules mode (GO111MODULE=on or outside GOPATH).

• The vendor directory is still populated, using go mod vendor. This means there is no difference in building stuff for those still using GOPATH etc. In the long run, Go 1.12+, we can drop the vendor dir.

• The non-Go dependencies we have for go generate (protobuf schemas), and the XDR builder, aren't easily accessible as modules. Instead I hacked the build script to clone and check out the stuff we need, if it isn't already present, when we run go run build.go proto. This isn't done very often by most developers, so I think it's fine.

• I removed the stuff in build.go for juggling GOPATH and building outside of it. If you're outside GOPATH today and on a modern compiler, the module support will kick in instead.

Testing

It still builds for me.

[calmh]

So there's a hilarious amount of protocol failures due to updating the protobuf package, so that's interesting and merits investigation...

[calmh]

This is not user facing, and should really only affect those juggling dependencies or wanting to work without gopath. Given that, is anyone strongly opposed to this change, @AudriusButkevicius @imsodin ?

[AudriusButkevicius]

This is probably the future, so I am ok with that. I am not planning to review the diff as I presume it's a massive noop change.

[imsodin]

Well, I knew how to deal with the oddities of gvt, now I guess I'll have to learn about go mod - I don't mind.

[calmh]

Welcome to the future. Now to fix all the unexpected build failures...