`essential loop detected` if multiple package-level essentials are given

When chisel consumes an SDF with multiple items defined in the package-level essentials, it errors out upon validation.

Example by modifying `slices/hello.yaml` from the `ubuntu-26.04` branch of chisel-releases:

```
package: hello

essential:
hello_copyright:
hello_bins:

slices:
bins:
essential:
libc6_libs:
contents:
/usr/bin/hello:

copyright:
contents:
/usr/share/doc/hello/copyright:
```

Run a cut command:

```
chisel cut --ignore=unstable --release ./ --root rootfs/ hello_bins
```

Chisel errors out:

```
2026/03/05 10:26:07 Processing ./ release...
error: essential loop detected: hello_bins, hello_copyright
```