Instead of having a separate implementation of the required
packages and the required lorax templates and similar we
now reuse the images `data/distrodefs` YAML so that all
the packages/definitions are maintained in a central place.

This commit adds a fallback check when no direct match for
the distro ID from the bootc image is found. As long as
the bootc container sets a correct `ID_LIKE` we should
automatically have the same behavior as before when
we used symlinks as aliases.

If needed we might need to reintroduce an explicit
mapping like:
```go
// mapping of distro IDs that are compatible with
var distroCompat = map[string]string{
	"almalinux":     "rhel",
	"aurora":        "fedora",
	"aurora-helium": "rhel",
	"bazzite":       "fedora",
	"bluefin":       "fedora",
	"heliumos":      "rhel",
	"rocky":         "rhel",
}
```
but lets hope we don't need this.

Note that these tests use the "real" distros.yaml from the images library
so make sure that if rhel-9 goes away the test is updated for rhel-XY
(only the version needs to change)

Reuse the helpers from the `images` library for to do:
- labelForISO()
- getDistroAndRunner()

This code moved into images as part of
osbuild/images#1906
and we can now reuse it here instead of duplicating it.

Workloads are no longer used in the images library. This
NullWorkload is also no longer used so drop it.

This commit merged manifestForISO and manifestFromCobraForLegacyISO
to improve readbility. Usually this is bad but here we have too
many *manifest*() functions and the benefit of having the extra
ManifestConfig struct is not there anymore. So drop all that so
that we only have:
```
manifestFromCobraForLegacyISO()
imgTypeManifestForLegacyISO()
```
left. The later will generate the `manifest.Maniest` only
and is like imgType.Manifest() (but legacyISO is not an imageType
yet).