Replace use of 'withAsync' by 'concurrently', to not lose exceptions

This fixes deadlock in asyncFetchPackages:

  withAsync action1 (\_ -> action2)

loses exceptions thrown by action1. If in addition, action2
has a dependency on data produced by action1 (as is the case
here), this will block indefinitely.

(This bug caused some of the new tests to hang, since after
the change to use async-safe try, it became easier for
fetchPackages to throw an exception.)

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

@@ -252,13 +252,15 @@ asyncFetchPackages verbosity repoCtxt pkglocs body = do
             -- Suppress marking here, because 'withAsync' means
             -- that we get nondeterministic interleaving.
             -- It is essential that we don't catch async exceptions here,
-            -- specifically AsyncCancelled thrown at us from withAsync.
+            -- specifically 'AsyncCancelled' thrown at us from 'concurrently'.
             result <- Safe.try $
               fetchPackage (verboseUnmarkOutput verbosity) repoCtxt pkgloc
             putMVar var result
 
-    withAsync fetchPackages $ \_ ->
-      body (Map.fromList asyncDownloadVars)
+    (_, res) <- concurrently
+        fetchPackages
+        (body $ Map.fromList asyncDownloadVars)
+    pure res
 
 
 -- | Expect to find a download in progress in the given 'AsyncFetchMap'