Use relative paths for haddockHTMLs in the inplace package.conf

This means that the docs get built with the correct relative paths.
When installing, the absolute path will still be used.

utils/ghc-cabal/ghc-cabal.hs

@@ -310,7 +310,10 @@ generate config_args distdir directory
               let ipid = InstalledPackageId (display (packageId pd) ++ "-inplace")
               let installedPkgInfo = inplaceInstalledPackageInfo cwd distdir
                                          pd lib lbi clbi
-                  final_ipi = installedPkgInfo{ Installed.installedPackageId = ipid }
+                  final_ipi = installedPkgInfo {
+                                  Installed.installedPackageId = ipid,
+                                  Installed.haddockHTMLs = ["../" ++ display (packageId pd)]
+                              }
                   content = Installed.showInstalledPackageInfo final_ipi ++ "\n"
               writeFileAtomic (distdir </> "inplace-pkg-config") content
           _ -> error "Inconsistent lib components; can't happen?"