remove gocart symlink
remove -a (aggregate) functionality

the workflow this was introduced for ended up being awkward. 'gocart check'
should replace this for getting the "big picture".

[finishes #64964782]
refactor dependency set management into Set object

represents a set of dependencies, covers loading Cartridge + Cartridge.lock,
and saving Cartridge.lock directories

also improved parsing; comments can now be left at the ends of lines, not just
on their own line
dependency fetcher -> fetcher