Do not allow `configure` to remove a definition when a instance depends on it

[vieiralucas]

Reproducing the bug

https://github.com/ergomake/layerform/assets/7764293/ef1e34cf-5b6f-4c33-ac1c-355739032917

1. Configure the local example:

layerform configure --file examples/local/layerform.json

2. Spawn an instance of bar

layerform spawn bar bar_instance

3. Update examples/local/layerform.json removing layer bar:

{
  "layers": [
    {
      "name": "foo",
      "files": ["foo.tf"]
    },
    {
      "name"  : "baz",
      "files": ["baz.tf"],
      "dependencies": ["foo"]
    }
  ]
}

4. Run configure again

layerform configure --file examples/local/layerform.json

Well, we just broke layerform, just trying to run layeform list instances will result in a panic, trying to kill bar_instance will also fail because it will not find the definition.

> layerform list instances                                
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x40 pc=0x1054f05c0]

goroutine 1 [running]:
github.com/ergomake/layerform/cmd/cli.computeDepth(0x1058a89a0?, 0x140001c5ae8?, 0x0)
        /Users/lucas/code/ergomake/layerform/cmd/cli/list_instances.go:111 +0x20
github.com/ergomake/layerform/cmd/cli.sortInstancesByDepth.func1(0x140001c58e8?, 0x1)
        /Users/lucas/code/ergomake/layerform/cmd/cli/list_instances.go:125 +0x68
sort.insertionSort_func({0x140001c59a0?, 0x14000141ec0?}, 0x0, 0x3)
        /opt/homebrew/Cellar/go/1.20.5/libexec/src/sort/zsortfunc.go:12 +0xc0
sort.stable_func({0x140001c59a0?, 0x14000141ec0?}, 0x3)
        /opt/homebrew/Cellar/go/1.20.5/libexec/src/sort/zsortfunc.go:343 +0x6c
sort.SliceStable({0x105856e40?, 0x1400012ced0?}, 0x4?)
        /opt/homebrew/Cellar/go/1.20.5/libexec/src/sort/slice.go:38 +0x104
github.com/ergomake/layerform/cmd/cli.sortInstancesByDepth({0x14000141e60, 0x3, 0x4}, 0x140001c5ae8)
        /Users/lucas/code/ergomake/layerform/cmd/cli/list_instances.go:122 +0x78
github.com/ergomake/layerform/cmd/cli.glob..func4(0x105e85c60?, {0x1054f29c3?, 0x0?, 0x0?})
        /Users/lucas/code/ergomake/layerform/cmd/cli/list_instances.go:82 +0x450
github.com/spf13/cobra.(*Command).execute(0x105e85c60, {0x105ec51b8, 0x0, 0x0})
        /Users/lucas/go/pkg/mod/github.com/spf13/[email protected]/command.go:944 +0x5b0
github.com/spf13/cobra.(*Command).ExecuteC(0x105e86220)
        /Users/lucas/go/pkg/mod/github.com/spf13/[email protected]/command.go:1068 +0x35c
github.com/spf13/cobra.(*Command).Execute(...)
        /Users/lucas/go/pkg/mod/github.com/spf13/[email protected]/command.go:992
github.com/ergomake/layerform/cmd/cli.Execute()
        /Users/lucas/code/ergomake/layerform/cmd/cli/root.go:30 +0xfc
main.main()
        /Users/lucas/code/ergomake/layerform/main.go:13 +0x144

Proposed solution

The second configure command should just fail in this situation, telling the user that it can't update configuration because it is trying to remove a definition that has an instance currently running.