custom package in go.mod

[Kushan-]

hey i followed your tutorial from medium, and it's amazing. I was looking for that since months. A quick question though how can i mention custom package in go.mod file.
my directory structure is :-

src
- pkg
-- geometry.go
main.go

import in main,go looks like

import (  
    "fmt"
	"./pkg/geometry" //importing custom package
	"github.com/gorilla/mux"
	"net/http"
)

I am always get error
build command-line-arguments: cannot find module for path _/app/src/pkg/geometry
I followed couple of documentation/answer in stack overflow, but replace statement ain't working for me.