Include signatures in SrcDist

Count signatures as modules that we have to find, and when searching for
modules, look for .hsig or .lhsig files (in addition to the usual .hs
.lhs files)

Cabal/Distribution/Simple/SrcDist.hs

@@ -156,8 +156,12 @@ listPackageSourcesOrdinary verbosity pkg_descr pps =
   [
     -- Library sources.
     fmap concat
-    . withAllLib $ \Library { exposedModules = modules, libBuildInfo = libBi } ->
-     allSourcesBuildInfo libBi pps modules
+    . withAllLib $ \Library {
+                      exposedModules = modules,
+                      signatures     = sigs,
+                      libBuildInfo   = libBi
+                    } ->
+     allSourcesBuildInfo libBi pps (modules ++ sigs)
 
     -- Executables sources.
   , fmap concat
@@ -437,7 +441,7 @@ allSourcesBuildInfo bi pps modules = do
   where
     nonEmpty x _ [] = x
     nonEmpty _ f xs = f xs
-    suffixes = ppSuffixes pps ++ ["hs", "lhs"]
+    suffixes = ppSuffixes pps ++ ["hs", "lhs", "hsig", "lhsig"]
     notFound m = die $ "Error: Could not find module: " ++ display m
                  ++ " with any suffix: " ++ show suffixes ++ ". If the module "
                  ++ "is autogenerated it should be added to 'autogen-modules'."