Fix relative imports for dhall freeze --inplace (#619)

Fixes https://github.com/dhall-lang/dhall-haskell/issues/617

This changes `dhall freeze` to use the directory of the supplied file path as
the root import instead of using ".".  This fixes the behavior of relative
imports when the root file is not located in the current working directory.

src/Dhall/Freeze.hs

@@ -27,16 +27,18 @@ import qualified Data.Text.IO
 import qualified Dhall.Core
 import qualified Dhall.Import
 import qualified Dhall.TypeCheck
+import qualified System.FilePath
 import qualified System.IO
 
-readInput :: Maybe FilePath -> IO Text
-readInput = maybe Data.Text.IO.getContents Data.Text.IO.readFile
-
 -- | Retrieve an `Import` and update the hash to match the latest contents
-hashImport :: ProtocolVersion -> Import -> IO Import
-hashImport _protocolVersion import_ = do
-    let status =
-            set protocolVersion _protocolVersion (Dhall.Import.emptyStatus ".")
+hashImport
+    :: FilePath
+    -- ^ Current working directory
+    -> ProtocolVersion
+    -> Import
+    -> IO Import
+hashImport directory _protocolVersion import_ = do
+    let status = set protocolVersion _protocolVersion (Dhall.Import.emptyStatus directory)
 
     expression <- State.evalStateT (Dhall.Import.loadWith (Embed import_)) status
 
@@ -90,9 +92,19 @@ freeze
     -> ProtocolVersion
     -> IO ()
 freeze inplace _protocolVersion = do
-    text <- readInput inplace
+    (text, directory) <- case inplace of
+        Nothing -> do
+            text <- Data.Text.IO.getContents
+
+            return (text, ".")
+
+        Just file -> do
+            text <- Data.Text.IO.readFile file
+
+            return (text, System.FilePath.takeDirectory file)
+
     (header, parsedExpression) <- parseExpr srcInfo text
-    frozenExpression <- traverse (hashImport _protocolVersion) parsedExpression
+    frozenExpression <- traverse (hashImport directory _protocolVersion) parsedExpression
     writeExpr inplace (header, frozenExpression)
         where
             srcInfo = fromMaybe "(stdin)" inplace