Multioutput stages have uniform type

[jbedo]

With the way we're doing filetype tagging (via passthru), all outputs of a derivation are tagged with the same type. This is OK for some cases (e.g., Xenomapper, which outputs a collection of BAM files) but not in others (Octopus calls variants and outputs a VCF file, but also produces a BAM evidence file as a side product). Some way of changing the type of secondary outputs is needed.