Recompute package registration AFTER we install (#3257).

It needs to be done afterwards, because we need to compute
the ABI hash as part of registration, which can't be done
unless all dependencies are registered, which could include
internal libraries.

This whole affair is very dodgy.
Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

cabal-install/Distribution/Client/Install.hs

@@ -1462,9 +1462,6 @@ installUnpackedPackage verbosity buildLimit installLock numJobs
 
         -- Install phase
           onFailure InstallFailed $ criticalSection installLock $ do
-            -- Capture installed package configuration file
-            maybePkgConf <- maybeGenPkgConf mLogPath
-
             -- Actual installation
             withWin32SelfUpgrade verbosity ipid configFlags
                                  cinfo platform pkg $ do
@@ -1474,6 +1471,11 @@ installUnpackedPackage verbosity buildLimit installLock numJobs
                   setup Cabal.copyCommand copyFlags mLogPath
                   when shouldRegister $ do
                     setup Cabal.registerCommand registerFlags mLogPath
+
+            -- Capture installed package configuration file
+            -- TODO: Why do we need this?
+            maybePkgConf <- maybeGenPkgConf mLogPath
+
             return (Right (BuildOk docsResult testsResult maybePkgConf))
 
   where