Managing Conflicting Proto File Packages and Import Paths in Go

Question

I have two proto files in the same folder.

Because of the history, a.proto's package name is different from b.proto. But they have a same message variable. Meanwhile, we can not change the defination because if we do this, there are too many files we need to change.

Firstly, the a.proto

package service_interface;
option go_package = "pb_gen/service";

message point {
    required uint32 x = 1;
    required uint32 y = 2;
}

message view {
    required uint32 x = 1;
    required uint32 y = 2;
}

then b.proto

import "a.proto";
package mydemo;
option go_package = "pb_gen/mydemo";

message point {
    required uint32 x = 1;
    required uint32 y = 2;
}
message bible {
    required service_interface.view v = 1;
}

And I run the command to generate the pb.go files, no errors. But when I import these two protos into my golang files,

import (
    "my/demo-api/pb_gen/mydemo"
)

it tells me

 package pb_gen/mydemo is not in GOROOT

I have set go.mod, looks like

module my/demo-api

What is the problem? Or is there a better way to manage these two proto files?