maps of deps are not reusable in deps.edn

Problem: :deps and other basis alias keys (:extra-deps etc) must be defined explicitly in deps.edn.

Example use case: would be to have a :dev alias that includes the :extra-deps from the :build alias.

Related: we have this indrection now for :paths in deps.edn (can source from an alias).

Lots of alternatives for how this could be syntactically expressed, need to consider impacts on merging and reading.