ghci: Explicitly store and restore interface file cache
In the old days the old HPT was used as an interface file cache when
using ghci. The HPT is a ModuleEnv HomeModInfo and so if you were
using hs-boot files then the interface file from compiling the .hs file
would be present in the cache but not the hi-boot file. This used to be
ok, because the .hi file used to just be a better version of the
.hi-boot file, with more information so it was fine to reuse it. Now the
source hash of a module is kept track of in the interface file and the
source hash for the .hs and .hs-boot file are correspondingly different
so it's no longer safe to reuse an interface file.
I took the decision to move the cache management of interface files to
GHCi itself, and provide an API where load can be provided with a list
of interface files which can be used as a cache. An alternative would be
to manage this cache somewhere in the HscEnv but it seemed that an API
user should be responsible for populating and suppling the cache rather
than having it managed implicitly.
Fixes #20217 (closed)
I am not sure this is the best approach, we can discuss how is best to solve this problem but this approach does work.