[cleanup] introduce filegen package, and move functions that generate files into it (#1189)

## Summary

This PR introduces a `filegen` package. It will contain functions
related to files that devbox generates in `.devbox/gen`:
- to enable `devbox.PrintEnv`, via `filegen.GenerateForPrintEnv`.
- to enable `devbox.RunScript` via `filegen.WriteScriptsToFile`.

My motivation was that I wanted to pull logic out of impl and plansdk
into a single package (and internalize parts of it so it is easier to
reason about or follow). Currently, we have logic for generating code in
.devbox/gen that is spread in different .go files in both impl and
plansdk.

No logic change. This is mostly consolidating code that has spread over
`impl/devbox.go` into a smaller package.


## How was it tested?

`devbox shell` worked.

- [x] testscripts.

Author: savil

internal/boxcli/generate/devcontainer_util.go

@@ -3,6 +3,9 @@
 
 package generate
 
+// TODO move this to package filegen at impl/filegen
+// no need to be in `boxcli`.
+
 import (
 	"context"
 	"embed"

internal/impl/generate.go renamed to internal/filegen/generate.go

@@ -1,7 +1,7 @@
 // Copyright 2023 Jetpack Technologies Inc and contributors. All rights reserved.
 // Use of this source code is governed by the license in the LICENSE file.
 
-package impl
+package filegen
 
 import (
 	"bytes"
@@ -17,26 +17,31 @@ import (
 	"text/template"
 
 	"github.com/pkg/errors"
-
 	"go.jetpack.io/devbox/internal/cuecfg"
 	"go.jetpack.io/devbox/internal/debug"
 	"go.jetpack.io/devbox/internal/planner/plansdk"
 )
 
+// TmplFS docblock to satisfy silly linter.
+// TODO savil: move package generate in boxcli into this filegen package, and then make `TmplFS` internal
+//
 //go:embed tmpl/*
-var tmplFS embed.FS
+var TmplFS embed.FS
 
 var shellFiles = []string{"shell.nix"}
 
-func (d *Devbox) generateShellFiles(ctx context.Context) error {
+// GenerateForPrintEnv will create all the files necessary for processing
+// devbox.PrintEnv, which is the core function from which devbox shell/run/direnv
+// functionality is derived.
+func GenerateForPrintEnv(ctx context.Context, devbox devboxer) error {
 	defer trace.StartRegion(ctx, "generateShellFiles").End()
 
-	plan, err := d.ShellPlan(ctx)
+	plan, err := devbox.FlakePlan(ctx)
 	if err != nil {
 		return err
 	}
 
-	outPath := filepath.Join(d.projectDir, ".devbox/gen")
+	outPath := filepath.Join(devbox.ProjectDir(), ".devbox/gen")
 
 	for _, file := range shellFiles {
 		err := writeFromTemplate(outPath, plan, file)
@@ -46,7 +51,7 @@ func (d *Devbox) generateShellFiles(ctx context.Context) error {
 	}
 
 	// Gitignore file is added to the .devbox directory
-	err = writeFromTemplate(filepath.Join(d.projectDir, ".devbox"), plan, ".gitignore")
+	err = writeFromTemplate(filepath.Join(devbox.ProjectDir(), ".devbox"), plan, ".gitignore")
 	if err != nil {
 		return errors.WithStack(err)
 	}
@@ -56,7 +61,7 @@ func (d *Devbox) generateShellFiles(ctx context.Context) error {
 		return errors.WithStack(err)
 	}
 
-	return d.writeScriptsToFiles()
+	return WriteScriptsToFiles(devbox)
 }
 
 // Cache and buffers for generating templated files.
@@ -76,7 +81,7 @@ func writeFromTemplate(path string, plan any, tmplName string) error {
 		tmpl.Funcs(templateFuncs)
 
 		var err error
-		tmpl, err = tmpl.ParseFS(tmplFS, "tmpl/"+tmplKey)
+		tmpl, err = tmpl.ParseFS(TmplFS, "tmpl/"+tmplKey)
 		if err != nil {
 			return errors.WithStack(err)
 		}

internal/impl/generate_test.go renamed to internal/filegen/generate_test.go

@@ -1,9 +1,10 @@
 // Copyright 2023 Jetpack Technologies Inc and contributors. All rights reserved.
 // Use of this source code is governed by the license in the LICENSE file.
 
-package impl
+package filegen
 
 import (
+	"flag"
 	"os"
 	"path/filepath"
 	"strings"
@@ -13,6 +14,9 @@ import (
 	"go.jetpack.io/devbox/internal/planner/plansdk"
 )
 
+// update overwrites golden files with the new test results.
+var update = flag.Bool("update", false, "update the golden files with the test results")
+
 func TestWriteFromTemplate(t *testing.T) {
 	dir := filepath.Join(t.TempDir(), "makeme")
 	outPath := filepath.Join(dir, "flake.nix")