An end result of #22674 is that `SkinBlueprintContainer`s are only ever
created by supplying a `SkinComponentsContainer` to them. However,
`SkinBlueprintContainer` still contained remnants of code that suggested
it was designed to handle cases where more than the drawable supplied to
it contained more than one `ISerialisableDrawableContainer`, or even
zero.
The zero path is totally dead right now (because every
`SkinComponentsContainer` is *by necessity* an
`ISerialisableDrawableContainer`), and the more-than-one path is dead
*for now* (and potentially forever?). Therefore, just hard-couple
`SkinBlueprintContainer` to receive a single target container.
