refactor(3p/overlays): Move haskell overlay to new tree location

nixpkgs_exposed is going away, and the haskell overlay is independent
from that.

See also b/108, cl/2910

Change-Id: I3aea6dfc427a914f3f88146fd0b45d60dfd45a1a
Reviewed-on: https://cl.tvl.fyi/c/depot/+/2918
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

third_party/overlays/haskell/patches/generic-arbitrary-export-garbitrary.patch

@@ -0,0 +1,12 @@
+diff --git a/src/Test/QuickCheck/Arbitrary/Generic.hs b/src/Test/QuickCheck/Arbitrary/Generic.hs
+index fed6ab3..91f59f1 100644
+--- a/src/Test/QuickCheck/Arbitrary/Generic.hs
++++ b/src/Test/QuickCheck/Arbitrary/Generic.hs
+@@ -23,6 +23,7 @@ The generated 'arbitrary' method is equivalent to
+ 
+ module Test.QuickCheck.Arbitrary.Generic
+   ( Arbitrary(..)
++  , GArbitrary
+   , genericArbitrary
+   , genericShrink
+   ) where