Re-enable flexible-defaults and others that it was breaking:

* `random-sources`
* `rvar`
* and `random-fu`

build-constraints.yaml

@@ -2527,9 +2527,9 @@ packages:
         - nix-paths
         - parsec-class
         - prim-uniq < 0 # via dependent-sum
-        - random-fu < 0 # via random-source
-        - random-source < 0 # via flexible-defaults
-        - rvar < 0 # via random-source
+        - random-fu
+        - random-source
+        - rvar
         - SafeSemaphore
         - streamproc < 0 # MonadFail
         - stringsearch # for cgi
@@ -4789,7 +4789,6 @@ packages:
     # be removed from this list if they are fixed.
     "Unmaintained packages with compilation failures":
         - doctest-discover-configurator < 0
-        - flexible-defaults < 0 # MonadFail
         - haskell-tools-builtin-refactorings < 0 # bounds failure
         - hpqtypes < 0 # bounds failure
         - stackage-types < 0