Support for using only partial pieces of included signatures.
Summary: Generally speaking, it's not possible to "hide" a requirement from a package you include, because if there is some module relying on that requirement, well, you can't just wish it out of existence. However, some packages don't have any modules. For these, we can validly thin out requirements; indeed, this is very convenient if someone has published a large signature package but you only want some of the definitions. This patchset tweaks the interpretation of export lists in signatures: in particular, they no longer need to refer to entities that are defined locally; they range over both the current signature as well as any signatures that were inherited from signature packages (defined by having zero exposed modules.) In the process of doing this, I cleaned up a number of other things: * rnModIface and rnModExports now report errors that occurred during renaming and can propagate these to the TcM monad. This is important because in the current semantics, you can thin out a type which is referenced by a value you keep; in this situation, we need to error (to ensure that all types in signatures are rooted, so that we can determine their identities). * I ended up introducing a new construct 'dependency signature; to bkp files, to make it easier to tell if we were depending on a signature package. It's not difficult for Cabal to figure this out (I already have a patch for it.) Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: simonpj, austin, bgamari Subscribers: thomie, mpickering Differential Revision: https://phabricator.haskell.org/D2904 GHC Trac Issues: #12994