Support for 'options go_package'

[idlebot]

go_proto_link currently copies the .pb.go files to the same directory as the .proto file.

However, if the .proto file has a options go_package defined, it does not copy to directory specified in the go_options.

For example, given a file:

proto/mypackage/v1/test.proto

package mypackage.v1;

option go_package = "github.com/org/repo/go/mypackage/v1;mypackagev1";

Ideally, it would copy test.pb.go to go/mypackage/v1 instead of proto/mypackage/v1.

One way that this could be accomplished would be by:

• define a importpath attribute in the go_proto_link rule just like go_proto_library.
• Gazelle plugin copies the importpath from go_proto_library
• go_proto_link rule uses the relevant part of importpath to copy to the correct destination directory, creating the path if needed.