print every subdir of libdir as a package

This is a quick and dirty solution for idris-lang#1071.

I wish I were able to implement output of metadata, but since there is
no metadata saved (currently) alongside of a package, there is nothing
to display.

Displaying metadata in the output of this subcommand would mean that
there have to happen further improvements in the package-system of
idris.

Please see my proposal in idris-lang#1126.

idris.cabal

@@ -794,6 +794,7 @@ Executable idris
  Build-depends: idris
  , base
  , filepath
+ , directory
  , haskeline >= 0.7
  , transformers
 

main/Main.hs

@@ -45,6 +45,7 @@ runIdris opts = do
  when (ShowIncs `elem` opts) $ runIO showIncs
  when (ShowLibs `elem` opts) $ runIO showLibs
  when (ShowLibdir `elem` opts) $ runIO showLibdir
+ when (ShowPkgs `elem` opts) $ runIO showPkgs
  case opt getClient opts of
  [] -> return ()
  (c:_) -> do setVerbose False
@@ -92,3 +93,10 @@ showIncs :: IO b
 showIncs = do incFlags <- getIncFlags
  putStrLn $ unwords incFlags
  exitWith ExitSuccess
+
+-- | List idris packages installed
+showPkgs :: IO b
+showPkgs = do dir <- getIdrisLibDir
+ pkgs <- getDirectoryContents dir
+ mapM putStrLn (filter (/= ".") . filter (/= "..") $ pkgs)
+ exitWith ExitSuccess