ghc-boot-th package reveals issue with build system's treatment of transitive dependencies

The ghc-boot-th introduced in the resolution of #12052 (closed) seems to have uncovered a bug in the build system's treatment of transitive dependencies. Namely, it seems to be possible for the build system to attempt to link ghc-pkg before libghc-boot-th has been produced (although this will typically only happen when building with -j1). This is odd since ghc-pkg has a dependency on ghc-boot, which in turn has a dependency on ghc-boot-th.

In the interest of moving the 8.0.1 release along I'm going to work around this for the time being by adding transitive dependencies to the rule in rules/build-prog.mk which builds programs (including ghc-pkg). It doesn't seem like this should be necessary, but with Hadian being so near it seems like a good enough work around.