Set/update upstream repo url for haddock

In the future, some script will use the last column to configure
Git to redirect `push` operations to the respective primary Git
repo.

One way could be via the `pushInsteadOf` facility described in

  https://ghc.haskell.org/trac/ghc/wiki/WorkingConventions/Git/Submodules

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>
Export `TimerManager` from GHC.Event (re #9165)

This just addresses the specific issue raised in #9165. However, I've noticed
the Haddock comments need to be improved, which will be addressed by a separate
commit.

Signed-off-by: Herbert Valerio Riedel <hvr@gnu.org>