Use plugin from `protobuf` dep, if any

[jeanparpaillon]

Whenever an application depends on protobuf and protobuf_compiler, the plugin should be generated from protobuf dep to perfectly match lib / compiler.

Rationale

sensible default, allow custom behaviour

Proposal

if protobuf found in deps do
  build from source
else
  if plug in PATH do
    check version and warn if no match
  else
    install plugin_version
  end
end

[jeanparpaillon]

@zolakeith wdyt ?