Don't crash on undefined names.

When we resolve an undfined name we record error, but we continue
processing things, so that we can report multiple errors.  As a result
we need to generate some sort of fake "real" name for the undefined entity.

Previously we used to generate a fake local name, but that confused parts
of the code that expected certain things to always be defined at the top
level.  So now we generate a fake name in the Cryptol prelude instead.

src/Cryptol/ModuleSystem/Renamer.hs

@@ -52,7 +52,7 @@ import Cryptol.Parser.AST
 import Cryptol.Parser.Selector(selName)
 import Cryptol.Utils.Panic (panic)
 import Cryptol.Utils.RecordMap
-import Cryptol.Utils.Ident(allNamespaces,OrigName(..),modPathCommon)
+import Cryptol.Utils.Ident(allNamespaces,OrigName(..),modPathCommon,preludeName)
 import Cryptol.Utils.PP
 
 import Cryptol.ModuleSystem.Interface
@@ -643,6 +643,7 @@ instance Rename TopDecl where
 renI :: Located (ImportG (ImpName PName)) ->
         RenameM (Located (ImportG (ImpName Name)))
 renI li =
+  withLoc (srcRange li)
   do m <- rename (iModule i)
      recordImport (srcRange li) m
      pure li { thing = i { iModule = m } }
@@ -838,7 +839,8 @@ renameType nt = resolveName nt NSType
 mkFakeName :: Namespace -> PName -> RenameM Name
 mkFakeName ns pn =
   do ro <- RenameM ask
-     liftSupply (mkLocal ns (getIdent pn) (roLoc ro))
+     liftSupply (mkDeclared ns (TopModule preludeName) SystemName (getIdent pn)
+                                                       Nothing (roLoc ro))
 
 -- | Rename a schema, assuming that none of its type variables are already in
 -- scope.

tests/modsys/nested/T16.cry

@@ -0,0 +1,2 @@
+import T16_M
+import submodule A

tests/modsys/nested/T16.icry

@@ -0,0 +1 @@
+:load T16.cry

tests/modsys/nested/T16.icry.stdout

@@ -0,0 +1,7 @@
+Loading module Cryptol
+Loading module Cryptol
+Loading module T16_M
+Loading module Main
+
+[error] at T16.cry:2:1--2:19
+    Module not in scope: A

tests/modsys/nested/T16_M.cry

@@ -0,0 +1,4 @@
+module T16_M where
+private
+  submodule A where
+    x = 0x02