Given this dependency structure:

• app
  • dep-a
    • dep-b

If dep-a and dep-b both have postinstall scripts, we want dep-b's postinstall to run first since dep-a may depend on it, just as we would if we ran shards build from within in dep-a's own directory locally.

I attempted to solve with #359 but it is not a guaranteed solution — since app and dep-a could both list dep-b as a dependency separately, its placement in the flattened list of dependencies is not guaranteed.