hadrian: Flavour: Change args -> extraArgs

Previously in a flavour definition you could override all the flags
which were passed to GHC. This causes issues when needed to compute a
package hash because we need to know what these extra arguments are
going to be before computing the hash. The solution is to modify flavour
so that the arguments you pass here are just extra ones rather than all
the arguments that you need to compile something.

This makes things work more like how cabal.project files work when you
give extra arguments to a package and also means that flavour
transformers correctly affect the hash.