Various fixes from Ben
- cabal files: Add missing headers to
extra-source-filesto ensure thatcabal sdistworks. - cabal files: Put
GhcDebug.cmmincmm-sourcesrather thanc-sources. This fixes the lexical errors we were seeing earlier. - cabal files: Drop references to non-existent files as they make
cabal v2-installupset. - Don't present that unpacked closures are GC pointers. They aren't and the GC will get confused if we say they are.