I observed a bug where if I modified the module which implemented
an hsig in another package, GHC would not recompile the signature
in this situation. The cause of this was two fold:
- calculateAvails is a bit subtle in that it doesn't actually set imp_mods; you have to set it yourself in the end.
- There was a bug with the module we passed to mkUsageInfos; it needs to be an identity module otherwise we exclude the implementing module from the list of depended upon modules.
Signed-off-by: Edward Z. Yang <email@example.com>