Duncan Coutts authored Sep 02, 2017 and Francesco Gazzetta committed Oct 14, 2017

So you can now add `extra-packages: foo` to the cabal.projct file and
then `cabal (new-)build foo`. The extra packages are included into the
install plan and they are also resolved as build targets.

Currently this only uses the "any valid package name" target syntax
which means you can use `foo` but not `foo:tests` or any of the other
variations.