Previously, the `make clean` (as well as `make dist-clean`) doesn't work
for ghc-heap and libiserv, due to these two libraries are not presented
in the "packages" file.

Test Plan: [skip ci]

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4878