[shell] minimal revert of #222 to bring back shell plans temporarily (#228)

## Summary

Until we can land #227, I want to revert the changes in #222 so that
shell plans can continue working, and we can release some other
(unrelated) fixes.

In this PR, I revert (manually) the `devbox.go` changes in #222, while
leaving the test changes to minimize merge conflicts with #227.

cc @loreto @LucilleH 

## How was it tested?

sanity check

```
> cd testdata/nodejs/nodejs-18
> devbox shell
> which node
# get nix store path
> exit

```

- [x] will find an example with init-hooks and test that.

Example 1: nginx

```
> devbox shell
Installing nix packages. This may take a while...done.
Starting a devbox shell...

##### WARNING: nginx planner is experimental #####

You may need to add

"include ./.devbox/gen/shell-helper-nginx.conf;"

to your shell-nginx.conf file to ensure the server can start in the nix shell.

Use "shell-nginx" to start the server
```

Example 2: python/pip-example

```
devbox/testdata/python/pip-example 
> devbox shell
Installing nix packages. This may take a while...done.
Starting a devbox shell...
Creating/Using virtual environment in /Users/savil/code/jetpack/devbox/testdata/python/pip-example/.venv

(devbox)
devbox/testdata/python/pip-example 
> which python3
/Users/savil/code/jetpack/devbox/testdata/python/pip-example/.venv/bin/python3
```

Author: savil

devbox.go

@@ -137,9 +137,13 @@ func (d *Devbox) Build(flags *docker.BuildFlags) error {
 
 // Plan creates a plan of the actions that devbox will take to generate its
 // shell environment.
-func (d *Devbox) ShellPlan() *plansdk.Plan {
-	// TODO: Move shell plan to a separate struct from build plan.
-	return d.convertToPlan()
+func (d *Devbox) ShellPlan() (*plansdk.Plan, error) {
+	userPlan := d.convertToPlan()
+	shellPlan, err := planner.GetShellPlan(d.srcDir)
+	if err != nil {
+		return nil, err
+	}
+	return plansdk.MergeUserPlan(userPlan, shellPlan)
 }
 
 // Plan creates a plan of the actions that devbox will take to generate its
@@ -172,13 +176,19 @@ func (d *Devbox) Shell() error {
 		return err
 	}
 
+	plan, err := d.ShellPlan()
+	if err != nil {
+		return errors.WithStack(err)
+	}
+
 	profileDir, err := d.profileDir()
 	if err != nil {
 		return err
 	}
 
 	nixShellFilePath := filepath.Join(d.srcDir, ".devbox/gen/shell.nix")
 	sh, err := nix.DetectShell(
+		nix.WithPlanInitHook(plan.ShellInitHook),
 		nix.WithProfile(profileDir),
 		nix.WithHistoryFile(filepath.Join(d.srcDir, shellHistoryFile)),
 	)
@@ -245,7 +255,14 @@ func (d *Devbox) convertToPlan() *plansdk.Plan {
 }
 
 func (d *Devbox) generateShellFiles() error {
-	return generate(d.srcDir, d.ShellPlan(), shellFiles)
+	shellPlan, err := d.ShellPlan()
+	if err != nil {
+		return err
+	}
+	if shellPlan.Invalid() {
+		return shellPlan.Error()
+	}
+	return generate(d.srcDir, shellPlan, shellFiles)
 }
 
 func (d *Devbox) generateBuildFiles() error {