Unify hsig and hs-boot; add preliminary "hs-boot" merging.
This patch drops the file level distinction between hs-boot and hsig; we figure out which one we are compiling based on whether or not there is a corresponding hs file lying around. To make the "import A" syntax continue to work for bare hs-boot files, we also introduce hs-boot merging, which takes an A.hi-boot and converts it to an A.hi when there is no A.hs file in scope. This will be generalized in Backpack to merge multiple A.hi files together; which means we can jettison the "load multiple interface files" functionality. This works automatically for --make, but for one-shot compilation we need a new mode: ghc --merge-requirements A will generate an A.hi/A.o from a local A.hi-boot file; Backpack will extend this mechanism further. Has Haddock submodule update to deal with change in msHsFilePath behavior. - This commit drops support for the hsig extension. Can we support it? It's annoying because the finder code is written with the assumption that where there's an hs-boot file, there's always an hs file too. To support hsig, you'd have to probe two locations. Easier to just not support it. - #10333 affects us, modifying an hs-boot still doesn't trigger recomp. - See compiler/main/Finder.hs: this diff is very skeevy, but it seems to work. - This code cunningly doesn't drop hs-boot files from the "drop hs-boot files" module graph, if they don't have a corresponding hs file. I have no idea if this actually is useful. Signed-off-by: Edward Z. Yang <firstname.lastname@example.org> Test Plan: validate Reviewers: simonpj, austin, bgamari, spinda Subscribers: thomie Differential Revision: https://phabricator.haskell.org/D1098