Fix up Cabal file setting for exe.

Some of the cabal files had settings that made it difficult/impossible to
initiate the project specific repl. This fix allows you to use:
```
$ cabal repl level0<N>
```
To start a repl for the library for that level.

Reworded the text for the ``Error`` type on level02 as it wasn't immediately
clear that you needed to add your own constructors.

Removed the default constructor for (``RqType``, ``Error``, ``ContentType``) to
try to make it more obvious that you have things to do there. Turned off the
'dodgy-imports' warning for that file specifically to allow for happy repl'ing.

level01/level01.cabal

@@ -67,7 +67,7 @@ library
   -- Base language which the package is written in.
   default-language:    Haskell2010
 
-executable level01
+executable level01-exe
   -- .hs or .lhs file containing the Main module.
   main-is:             Main.hs
 

level02/level02.cabal

@@ -72,7 +72,7 @@ library
   default-language:    Haskell2010
 
 
-executable level02
+executable level02-exe
   -- .hs or .lhs file containing the Main module.
   main-is:             Main.hs
 

level02/src/FirstApp/Types.hs

@@ -1,4 +1,5 @@
-{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE OverloadedStrings          #-}
+{-# OPTIONS_GHC -fno-warn-dodgy-exports #-}
 module FirstApp.Types
   ( Topic
   , CommentText
@@ -23,24 +24,23 @@ import           Data.Text       (Text)
 -- - View a topic and its comments
 -- - List the current topics
 
--- To that end, we will create the following constructors for our RqType:
+-- To that end, create the following constructors for our RqType:
 
 -- AddRq : Which needs the target topic, and the body of the comment.
 -- ViewRq : Which needs the topic being requested.
 -- ListRq : Which doesn't need anything and lists all of the current topics.
-data RqType = RqType
+data RqType
 
 -- Not everything goes according to plan, but it's important that our types
 -- reflect when errors can be introduced into our program. Additionally it's
 -- useful to be able to be descriptive about what went wrong.
 
--- Leave this type empty for now, and we'll fill in the error constructors we
--- need as we progress through the application.
-data Error = Error
+-- Fill in the error constructors as you need them.
+data Error
 
--- Provide a type to list our response content types so we don't try to do the
--- wrong thing with what we meant to be used as text or JSON etc.
-data ContentType = ContentType
+-- Provide the constructors for a sum type to specify the `ContentType` Header,
+-- to be used when we build our Response type.
+data ContentType
 
 -- The ``ContentType`` constructors don't match what is required for the header
 -- information, so write a function that will take our ``ContentType`` and

level03/level03.cabal

@@ -74,7 +74,7 @@ library
   -- Base language which the package is written in.
   default-language:    Haskell2010
 
-executable level03
+executable level03-exe
   -- .hs or .lhs file containing the Main module.
   main-is:             Main.hs
 

level04/level04.cabal

@@ -75,7 +75,7 @@ library
   -- Base language which the package is written in.
   default-language:    Haskell2010
 
-executable level04
+executable level04-exe
   -- .hs or .lhs file containing the Main module.
   main-is:             Main.hs