morphloader: Require systems have at least one stratum

It doesn't currently make sense to build a system which contains no
strata. We may later add other fields, such as initramfs to contribute
to the system's artifact, but until then it's another bug to trip over.

This uses collections.Sequence for checking the type of the systems entry
in the morphology as a style choice, though it allows more flexibility
if the types in the parsed morphology change.

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