There are multiple hacks all over the build system to account for the
fact that the ghc package uses different build subdirectories
(stage1/stage2) than the other packages (dist/dist-install).

One such hack filtered on 'ghc%', with the intention of filtering the
ghc package only. After renaming bin-package-db to ghc-boot
(d2f9972a, Phab:D1313, #10796), ghc-boot
also got caught in the hack, which broke the build when running without
parallelism.

This patch replaces the before mentioned hack by a different one, such
that filtering on 'ghc%' is no longer necessary. See Note [inconsistent
distdirs].

Reviewed by: austin

Differential Revision: https://phabricator.haskell.org/D1333