build: strip GOPATH from build paths

[Stebalien]

This should help fix some of our problems using plugins with go-ipfs.

[Kubuxu]

How will that work with out of GOPATH builds?

[Stebalien]

How will that work with out of GOPATH builds?

It will still strip the paths from the go modules.

I'm trying to make it easier to build plugins for go-ipfs. Unfortunately, it looks like go barfs if the paths differ (:anger:). My plan is to:

• Strip the paths when building plugins.
• Strip the paths when building go-ipfs with make. This won't completely normalize the go-ipfs paths, but will normalize all the dependencies.
• Build the dists with go mod (Build directly with go mod distributions#240)

All together, this will mean:

1. If we're building against a local copy of go-ipfs by using a "replace" directive in the plugin's go.mod, all the paths will match (all stripped except maybe the go-ipfs one).
2. If we're building against a published version of go-ipfs, all the paths will match (all stripped).

We should also consider extracting the plugin interfaces so plugins don't have to depend on go-ipfs itself. That should make this even simpler.

[Kubuxu]

We can make plugin interface a separate go module (with separate versioning) while keeping it in go-ipfs repo.