hadrian: Set -this-package-name

When constructing the GHC flags for a package Hadrian must take care to
set `-this-package-name` in addition to `-this-unit-id`. This hasn't
broken until now as we have not had any uses of qualified package
imports. However, this will change with `filepath-1.5` and the
corresponding `unix` bump, breaking `hadrian/multi-ghci`.