Support for source_relative argument

[octo47]

We are using source_relative for go_out plugin.
Effect of this use is that generated go files put in same directory as .proto file.
Unfortunately grpc-gateway generator has not such option and so it generates files using full go_package.

For example file echo.proto sitting in /protocol/echo:

package echo;
option go_package = "github.com/mycorp/repo.git/protocol/echo";

go_out supports generation of source_relative which means .go files will be put along in a same directory /protocol/echo.
This is not the case for grpc-gateway generator, files are put into /protocol/github.com/mycorp/repo.git/protocol/echo

Would be nice to support same source_relative argument as go generator does.

Previously workaround was to use simple package names like go_package="echo", but now as per 1.4rc it is deprecated feature and will be not supported soon.

2020/03/20 10:56:59 WARNING: Deprecated use of 'go_package' option without a full import path in "echo/echo_service.proto", please specify:
	option go_package = "echo";
A future release of protoc-gen-go will require the import path be specified.
See https://developers.google.com/protocol-buffers/docs/reference/go-generated#package for more information.

[johanbrandhorst]

I think we do support the paths=source_relative parameter, have you tried it?

[octo47]

Oh right! Sorry for long response.

[octo47]

Thank you for pointing this out.