How to use cue/load to load CUE files from memory

[jpluscplusm]

From https://cuelang.slack.com/archives/C012UU8B72M/p1721323828896049

go mod tidy
go run .
cmp stdout stdout.golden

-- go.mod --
module mod.example

go 1.22.3

require cuelang.org/go v0.9.2

-- main.go --
package main

import (
	"fmt"
	"log"
	"os"
	"path/filepath"

	"cuelang.org/go/cue/cuecontext"
	"cuelang.org/go/cue/load"
)

func main() {
	td, err := os.MkdirTemp("", "")
	if err != nil {
		log.Fatal(err)
	}
	defer os.RemoveAll(td)

	overlay := map[string]load.Source{
		// cue.mod/module.cue file
		filepath.Join(td, "cue.mod", "module.cue"): load.FromString(`
module: "mod.example"
language: version: "v0.9.2"
	`),

		// File in root
		filepath.Join(td, "a.cue"): load.FromString(`
package a

a: 5
	`),

		// File in subdirectory (that does not exist)
		filepath.Join(td, "b", "b.cue"): load.FromString(`
package a

b: 4
	`),
	}

	ctx := cuecontext.New()
	bis := load.Instances([]string{".", "./b"}, &load.Config{
		Dir:     td,
		Overlay: overlay,
	})
	for _, bi := range bis {
		v := ctx.BuildInstance(bi)
		fmt.Printf("%s: %v\n", bi.ImportPath, v)
	}
}
-- stdout.golden --
mod.example@v0:a: {
	a: 5
}
mod.example/b@v0:a: {
	a: 5
	b: 4
}

"The root directory needs to exist on disk, yes, but none of its contents do. So assuming you can use a temporary directory in this way, then nothing actually needs to be written to disk."