Perhaps the comment in this line of code describes the issue best:

An import path may not actually have a slash in it. For example if you define a vanity import such as "go.mycompany.com" and then define your protobuf file at the root of the module as such:

syntax = "proto3";
package mycompany;
option go_package = "go.mycompany.com;mycompany";

...

Twirp will end up generating the following Go package declaration:

package go_mycompany_com_mycompany

...

This is clearly incorrect. Twirp should instead see if there's a semi-colon in the go_package option and then return the left hand side as the import path and the right hand side as the package name.