ghc-pkg: Munge dynamic library directories

Otherwise we end up looking in the wrong place for dynamic libraries on
Windows. This addresses a regression introduced by D2611. See #12479.

Test Plan: validate across platforms

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2640

GHC Trac Issues: #12479

utils/ghc-pkg/Main.hs

@@ -822,6 +822,7 @@ mungePackagePaths top_dir pkgroot pkg =
       importDirs  = munge_paths (importDirs pkg),
       includeDirs = munge_paths (includeDirs pkg),
       libraryDirs = munge_paths (libraryDirs pkg),
+      libraryDynDirs = munge_paths (libraryDynDirs pkg),
       frameworkDirs = munge_paths (frameworkDirs pkg),
       haddockInterfaces = munge_paths (haddockInterfaces pkg),
                      -- haddock-html is allowed to be either a URL or a file