When parsing ghc-pkg output, only look at the first package.conf file for...

When parsing ghc-pkg output, only look at the first package.conf file for GlobalPackageDB and SpecificPackageDB, and all package.conf files for UserPackageDB.
Before, we would consider user packages when fulfilling dependencies
for global installs. ghc-pkg will refuse to install packages globally if they
use user packages. Thus, without this patch, global installs can fail when you have user packages installed.

Distribution/Simple/GHC.hs

@@ -226,8 +226,9 @@ getInstalledPackages verbosity packagedb conf = do
    --TODO: use --simple-output flag for easier parsing
    str <- rawSystemProgramStdoutConf verbosity ghcPkgProgram conf
             [packageDbGhcPkgFlag packagedb, "list"]
-   let keep_line s = ':' `notElem` s && not ("Creating" `isPrefixOf` s)
-       str1 = unlines (filter keep_line (lines str))
+   let str1 = case packagedb of
+                UserPackageDB -> allFiles str
+                _             -> firstFile str
        str2 = filter (`notElem` ",(){}") str1
        --
    case pCheck (readP_to_S (many (skipSpaces >> parsePackageId)) str2) of
@@ -241,6 +242,13 @@ getInstalledPackages verbosity packagedb conf = do
     pCheck :: [(a, [Char])] -> [a]
     pCheck rs = [ r | (r,s) <- rs, all isSpace s ]
 
+    allFiles str = unlines $ filter keep_line $ lines str
+        where keep_line s = ':' `notElem` s && not ("Creating" `isPrefixOf` s)
+
+    firstFile str = unlines $ takeWhile (not . file_line) $
+                    drop 1 $ dropWhile (not . file_line) $ lines str
+        where file_line s = ':' `elem` s && not ("Creating" `isPrefixOf` s)
+
 -- -----------------------------------------------------------------------------
 -- Building