MAINT: ensure that input names are consistent across actions

[gregcaporaso]

This is very minor, but might be worth addressing before an alpha release.

These two actions in assembly use different names for the same input type. It might be nice to pick one and use that for all actions in the plugin for consistency. This happens all over the place in the core distro, but it would be a breaking change to address it, so it hasn't been worth the trouble.

qiime assembly assemble-megahit --i-seqs demux.qza ...

qiime assembly map-reads-to-contigs --i-reads demux.qza ...