Support Go modules: fix protobuf imports

[alpe]

Is your feature request related to a problem? Please describe.
With the recent introduction of Go modules the project must not live within the Gopath anymore.

As of Go 1.11, the go command enables the use of modules when the current directory or any parent directory has a go.mod, provided the directory is outside $GOPATH/src. (Inside $GOPATH/src, for compatibility, the go command still runs in the old GOPATH mode, even if a go.mod is found.

https://blog.golang.org/using-go-modules

While the Go code in the project works outside the GOPATH now the protobuf generation does not as files are depending on parent path structures.

Describe the solution you'd like
Make it fully work outside GOPATH as intended by the modules designers

Describe alternatives you've considered

Additional context
go list -f '{{ .Dir }}' -m github.com/golang/protobuf may become helpful to set a proper imports in the makefile. See https://www.reddit.com/r/golang/comments/9blgn0/using_go_modules_with_vendorprovided_protobuf/

[alpe]

With make deps the local vendor folder is created and it works outside the GOPATH as requested. I will close this now.

[alpe]

Na, does not work with the imports to other weave components... 😭