warning when offline mode skips source-repository-packages

i.e.
```
Warning: --offline was specified, skipping sync of repositories:
Warning: https://github.com/haskell/text.git
```

cabal-install/src/Distribution/Client/ProjectConfig.hs

@@ -1374,13 +1374,16 @@ syncAndReadSourcePackagesRemoteRepos
         -- For syncing we don't care about different 'SourceRepo' values that
         -- are just different subdirs in the same repo.
         -- Do not sync source repositories when `--offline` flag applied.
-        unless offlineMode $
+        if not offlineMode then
           syncSourceRepos
             verbosity
             vcs
             [ (repo, repoPath)
             | (repo, _, repoPath) <- repoGroupWithPaths
             ]
+        else do
+          liftIO . warn verbosity $ "--offline was specified, skipping sync of repositories:"
+          liftIO . for_ repoGroupWithPaths $ \(repo,_,_) -> warn verbosity $ srpLocation repo
 
         -- Run post-checkout-command if it is specified
         for_ repoGroupWithPaths $ \(repo, _, repoPath) ->