Allow imports of internal modules

changelog_begin
- [Daml Compiler] Imports of internal modules are no longer illegal.
  Previously, the compiler raised an error when it encountered imports
  of internal modules such as `DA.Internal.*`. Such imports are now
  accepted by the compile. Note, however, that internal modules are
  still not part of the stable API.
  Fixes #10379
changelog_end

Previously, imports of internal modules such as

compiler/damlc/daml-preprocessor/src/DA/Daml/Preprocessor.hs

@@ -100,8 +100,7 @@ damlPreprocessor dataDependableExtensions mbUnitId dflags x
             , checkKinds x
             ]
         , preprocErrors = concat
-            [ checkImports x
-            , checkDataTypes x
+            [ checkDataTypes x
             , checkModuleDefinition x
             , checkRecordConstructor x
             , checkModuleName x
@@ -160,12 +159,6 @@ checkModuleName (GHC.L _ m)
     | otherwise
     = []
 
--- | We ban people from importing modules such
-checkImports :: GHC.ParsedSource -> [(GHC.SrcSpan, String)]
-checkImports x =
-    [ (ss, "Import of internal module " ++ GHC.moduleNameString m ++ " is not allowed.")
-    | GHC.L ss GHC.ImportDecl{ideclName=GHC.L _ m} <- GHC.hsmodImports $ GHC.unLoc x, isInternal m]
-
 -- | Emit a warning if the "daml 1.2" version header is present.
 checkDamlHeader :: GHC.ParsedSource -> [(GHC.SrcSpan, String)]
 checkDamlHeader (GHC.L _ m)

compiler/damlc/tests/daml-test-files/InternalImport.daml

@@ -1,9 +1,8 @@
 -- Copyright (c) 2020, Digital Asset (Switzerland) GmbH and/or its affiliates.
 -- All rights reserved.
 
--- @ERROR range=9:1-9:28; Import of internal module DA.Internal.Template is not allowed.
-
+-- Internal imports used to be forbidden but are allowed since 1.16.0.
 
 module InternalImport where
 
-import DA.Internal.Template
+import DA.Internal.Template()