proposal: Support simple code that contain 3rd package in go run rather than need a go.mod

[NewUserHa]

eg: any simple code that's only few lines and a single file like:

# test.go
package main

import (
	"fmt"
	"net/http"

	"github.com/gin-gonic/gin"
)

func main() {
	fmt.Println("123")
	r := gin.Default()
	r.GET("/ping", func(c *gin.Context) {
		c.JSON(http.StatusOK, gin.H{
			"message": "pong",
		})
	})
	r.Run() // listen and serve on 0.0.0.0:8080 (for windows "localhost:8080")
}

go run test.go will get "test.go:7:2: no required module provides package github.com/gin-gonic/gin: go.mod file not found in current directory or any parent directory; see 'go help modules'", which is technically correct in the current latest version.

But what about adding the default package searching paths including the two folders more: the $goroot and the current folder, to make those single .go files with simple codes that have 1 or 2 3rd party packages could run without any problem? (it currently can run when only the std libs.)

The change affected things may be:
go run test.go # this one currently will only work if only using std libs.
go get "github.com/gin-gonic/gin" # this one currently will complain like "... outside go mod..." and refuse work.

[seankhliao]

Duplicate of #36513