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 <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: simonpj, austin, bgamari, spinda

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D1098

2 files changed, 4 insertions, 3 deletions

diff --git a/utils/ghctags/Main.hs b/utils/ghctags/Main.hs
index a377953b38..4062535c05 100644
--- a/utils/ghctags/Main.hs
+++ b/utils/ghctags/Main.hs
@@ -224,9 +224,9 @@ fileTarget filename = Target (TargetFile filename Nothing) True Nothing
 graphData :: ModuleGraph -> (Maybe Handle, Maybe Handle) -> Ghc ()
 graphData graph handles = do
     mapM_ foundthings graph
-    where foundthings ms =
-              let filename = msHsFilePath ms
-                  modname = moduleName $ ms_mod ms
+    where foundthings ms
+            | Just filename <- msHsFilePath ms =
+              let modname = moduleName $ ms_mod ms
               in handleSourceError (\e -> do
                                        printException e
                                        liftIO $ exitWith (ExitFailure 1)) $
@@ -238,6 +238,7 @@ graphData graph handles = do
                          liftIO (writeTagsData handles =<< fileData filename modname s)
                        _otherwise ->
                          liftIO $ exitWith (ExitFailure 1)
+            | otherwise = return ()
 
 fileData :: FileName -> ModuleName -> RenamedSource -> IO FileData
 fileData filename modname (group, _imports, _lie, _doc) = do
diff --git a/utils/haddock b/utils/haddock
-Subproject fea4277692ba68cccc6c9642655289037e4b897
+Subproject 5890a2d503b3200e9897ce331ad61d808a67fca