ModIface passed to interface plugins have bottom fields for unclear reasons

Plugin.interfaceLoadAction is called after loading an interface by loadInterface.

loadInterface updates some fields of the ModIface before adding it to the PackageIfaceTable (eps_PIT), making them bottoming:

        ; let { final_iface = iface {
                                mi_decls     = panic "No mi_decls in PIT",
                                mi_insts     = panic "No mi_insts in PIT",
                                mi_fam_insts = panic "No mi_fam_insts in PIT",
                                mi_rules     = panic "No mi_rules in PIT",
                                mi_anns      = panic "No mi_anns in PIT"
                              }
               }

        ...
          updateEps_  $ \ eps -> ... add final_iface to PIT ...

        ; -- invoke plugins
          res <- withPlugins dflags interfaceLoadAction final_iface
        ; return (Succeeded res)
        }

There are two problems with this:

• Whatever the motivation for clearing these fields out is, it's not documented in the relevant places. A plugin author using this callback will probably look at the ModIface documentation, but not at EPS documentation, which is the only place where we mention that these fields are bottom:

  eps_PIT :: !PackageIfaceTable,
          -- ^ The 'ModIface's for modules in external packages
          -- whose interfaces we have opened.
          -- The declarations in these interface files are held in the
          -- 'eps_decls', 'eps_inst_env', 'eps_fam_inst_env' and 'eps_rules'
          -- fields of this record, not in the 'mi_decls' fields of the
          -- interface we have sucked in.

• Passing values with bottom fields to plugins is bad from UX point of view.

The same bottom fields caused other problems to me before, in a different context. See #17470.

I propose one of these two:

• Even in EPS documentation I don't see an explanation of why we need to clear these fields. Just keep them. We already keep the values alive in EPS, removing them from ModIface won't save us memory or anything else.

• Pass ModIface without removing these fields to the plugins so that a plugin can easily look at the annotations, rules etc. without having to go through undocumented stuff. Note that if we absolutely need to remove these from ModIface then we can still do it, this doesn't prevent us from removing the fields in the ModIface we put into the EPS.