cabal-install: allow Basic authentication in curl transport

Allow the curl transport to use Basic authentication, if and only if
the url scheme is HTTPS (i.e. TLS will be used).  Retain the
existing behaviour (force Digest scheme) for insecure requests.

This change is required to support upcoming hackage-server changes.
The wget transport already supports Basic authentication.

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

@@ -520,12 +520,18 @@ curlTransport prog =
             (Just (Left (uname, passwd)), _) -> Just $ Left (uname ++ ":" ++ passwd)
             (Nothing, Just a) -> Just $ Left a
             (Nothing, Nothing) -> Nothing
+      let authnSchemeArg
+            -- When using TLS, we can accept Basic authentication.  Let curl
+            -- decide based on the scheme(s) offered by the server.
+            | isHttpsURI uri = "--anyauth"
+            -- When not using TLS, force Digest scheme
+            | otherwise = "--digest"
       case mbAuthStringToken of
         Just (Left up) ->
           progInvocation
             { progInvokeInput =
                 Just . IODataText . unlines $
-                  [ "--digest"
+                  [ authnSchemeArg
                   , "--user " ++ up
                   ]
             , progInvokeArgs = ["--config", "-"] ++ progInvokeArgs progInvocation