When a cabal file can't be found, warn that extra-deps could be the p…

…roblem too.

* Also: slight refactor to use only `concat` instead of ++ in the error message
* Also: mention that a package.yaml file would also be accepted
	* The code that throws this is only searching for a .cabal file, but it runs hpack on the directory immediately before that to generate a cabal file, so it's effectively looking for a package.yaml file as well

Closes #3806

src/Stack/Types/Package.hs

@@ -51,9 +51,10 @@ instance Show PackageException where
         ]
     show (PackageNoCabalFileFound dir) = concat
         [ "Stack looks for packages in the directories configured in"
-        , " the 'packages' variable defined in your stack.yaml\n"
-        , "The current entry points to " ++ toFilePath dir ++
-          " but no .cabal file could be found there."
+        , " the 'packages' and 'extra-deps' fields defined in your stack.yaml\n"
+        , "The current entry points to "
+        , toFilePath dir
+        , " but no .cabal or package.yaml file could be found there."
         ]
     show (PackageMultipleCabalFilesFound dir files) =
         "Multiple .cabal files found in directory " ++