Refactoring to use cabal-helper-wrapper

This turned out to be quite involved but save for this huge commit it's
actually quite awesome and squashes quite a few bugs and nasty
problems (hopefully). Most importantly we now have native cabal
component support without the user having to do anything to get it!

To do this we traverse imports starting from each component's
entrypoints (library modules or Main source file for executables) and
use this information to find which component's options each module will
build with. Under the assumption that these modules have to build with
every component they're used in we can now just pick one.

Quite a few internal assumptions have been invalidated by this
change. Most importantly the runGhcModT* family of cuntions now change
the current working directory to `cradleRootDir`.

Language/Haskell/GhcMod/FillSig.hs

@@ -19,8 +19,10 @@ import qualified GHC as G
 import qualified Name as G
 import qualified Language.Haskell.GhcMod.Gap as Gap
 import Language.Haskell.GhcMod.Convert
+import Language.Haskell.GhcMod.DynFlags
 import Language.Haskell.GhcMod.Monad
 import Language.Haskell.GhcMod.SrcUtils
+import Language.Haskell.GhcMod.Doc
 import Language.Haskell.GhcMod.Types
 import Outputable (PprStyle)
 import qualified Type as Ty
@@ -66,21 +68,29 @@ sig :: IOish m
     -> Int          -- ^ Line number.
     -> Int          -- ^ Column number.
     -> GhcModT m String
-sig file lineNo colNo = ghandle handler body
-  where
-    body = inModuleContext file $ \dflag style -> do
-        opt <- options
-        modSum <- Gap.fileModSummary file
-        whenFound opt (getSignature modSum lineNo colNo) $ \s -> case s of
+sig file lineNo colNo =
+    runGmLoadedT' [Left file] deferErrors $ ghandle handler $ do
+      opt <- options
+      style <- getStyle
+      dflag <- G.getSessionDynFlags
+      modSum <- Gap.fileModSummary file
+      whenFound opt (getSignature modSum lineNo colNo) $ \s ->
+        case s of
           Signature loc names ty ->
-            ("function", fourInts loc, map (initialBody dflag style ty) names)
-          InstanceDecl loc cls ->
-             ("instance", fourInts loc, map (\x -> initialBody dflag style (G.idType x) x)
-                                            (Ty.classMethods cls))
+              ("function", fourInts loc, map (initialBody dflag style ty) names)
+
+          InstanceDecl loc cls -> let
+                body x = initialBody dflag style (G.idType x) x
+            in
+              ("instance", fourInts loc, body `map` Ty.classMethods cls)
+
           TyFamDecl loc name flavour vars ->
             let (rTy, initial) = initialTyFamString flavour
-             in (rTy, fourInts loc, [initial ++ initialFamBody dflag style name vars])
+                body = initialFamBody dflag style name vars
+             in (rTy, fourInts loc, [initial ++ body])
 
+
+  where
     handler (SomeException _) = do
       opt <- options
       -- Code cannot be parsed by ghc module
@@ -321,10 +331,11 @@ refine :: IOish m
        -> Int          -- ^ Column number.
        -> Expression   -- ^ A Haskell expression.
        -> GhcModT m String
-refine file lineNo colNo expr = ghandle handler body
-  where
-    body = inModuleContext file $ \dflag style -> do
+refine file lineNo colNo expr =
+  runGmLoadedT' [Left file] deferErrors $ ghandle handler $ do
         opt <- options
+        style <- getStyle
+        dflag <- G.getSessionDynFlags
         modSum <- Gap.fileModSummary file
         p <- G.parseModule modSum
         tcm@TypecheckedModule{tm_typechecked_source = tcs} <- G.typecheckModule p
@@ -339,7 +350,8 @@ refine file lineNo colNo expr = ghandle handler body
                   text = initialHead1 expr iArgs (infinitePrefixSupply name)
                in (fourInts loc, doParen paren text)
 
-    handler (SomeException _) = emptyResult =<< options
+ where
+   handler (SomeException _) = emptyResult =<< options
 
 -- Look for the variable in the specified position
 findVar :: GhcMonad m => DynFlags -> PprStyle
@@ -386,10 +398,11 @@ auto :: IOish m
      -> Int          -- ^ Line number.
      -> Int          -- ^ Column number.
      -> GhcModT m String
-auto file lineNo colNo = ghandle handler body
-  where
-    body = inModuleContext file $ \dflag style -> do
+auto file lineNo colNo =
+  runGmLoadedT' [Left file] deferErrors $ ghandle handler $ do
         opt <- options
+        style <- getStyle
+        dflag <- G.getSessionDynFlags
         modSum <- Gap.fileModSummary file
         p <- G.parseModule modSum
         tcm@TypecheckedModule {
@@ -415,8 +428,8 @@ auto file lineNo colNo = ghandle handler body
           djinns <- djinn True (Just minfo) env rty (Max 10) 100000
           return ( fourInts loc
                  , map (doParen paren) $ nub (djinnsEmpty ++ djinns))
-
-    handler (SomeException _) = emptyResult =<< options
+ where
+   handler (SomeException _) = emptyResult =<< options
 
 -- Functions we do not want in completions
 notWantedFuns :: [String]