dev: refactor sources.txt
dev: refactor Makefile

blog/sources.txt (for cabal-meta) no longer pulls in dependencies. i'm
not really sure how i'll end up using cabal-meta. ideally i should be
using it as a drop-in replacement for 'cabal install'. but what about
'cabal build'? i think it's best to use cabal-meta for ensuring the
dependencies are installed, and the Makefile uses it internally