Use go 1.11 module versioning support

Go 1.11 is adding preliminary support for Go modules, replacing the GOPATH-based approach.

obfs4 is adding go.mod and go.sum files, which use the Go modules support to fetch the dependencies. We disable that in legacy/trac#28310 (moved) as the go libraries we build are not currently recognized as Go modules.

We should change how we build our Go libraries so that they can be used as Go modules.

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