Lift the restriction that libraries must have exposed-modules

This allows libs that have only private modules or C code. This
might be used to make libs that have non-exposed modules and only
export C APIs. It could also be used to make packages that consist
only of C code. That might be useful for bindings where it may
make sense to split the C and Haskell code into separate packages.

Distribution/PackageDescription/Check.hs

@@ -189,11 +189,7 @@ checkLibrary :: Library -> [PackageCheck]
 checkLibrary lib =
   catMaybes [
 
-    check (buildable (libBuildInfo lib) && null (exposedModules lib)) $
-       PackageBuildImpossible
-         "A library was specified, but no 'exposed-modules' list has been given."
-
-  , check (not (null moduleDuplicates)) $
+    check (not (null moduleDuplicates)) $
        PackageBuildWarning $
          "Duplicate modules in library: " ++ commaSep moduleDuplicates
   ]