Make the Plutus Tx plugin an internal library

The documentation for `addCorePlugin` says it has to be in a separate
package to the plugin. However it appears that in GHC's eyes internal
libraries in a Cabal package are separate packages, so we can hide all
of this in a single package, which is much nicer.

We then have to do some shennanigans to get doctest to work. The normal
tests work fine, since Cabal registers the built internal library as a
package, and that's fine. But doctest interprets things, which *doesn't*
work.

Now, I initially tried to just pass the appropriate `-package-db`
arguments to `doctest`, but in doing so I hit what appears to be a [Cabal
bug with internal libraries](haskell/cabal#5857).
However, somehow `cabal repl` and friends seem to get around this, and
fortunately there is now a `cabal doctest` command, which amazingly
also seems to pass the right set of obscure flags. So we use that.

Finally, in an extremely irritating discovery, apparently [Haddock
doesn't work with internal libraries]
(haskell/cabal#4969) (possibly fixed in a
newer Cabal, who knows), so I ended up having to exclude `plutus-tx`
from Haddock generation for now.

cabal.project

@@ -3,15 +3,13 @@ packages: language-plutus-core
           plutus-exe
           wallet-api
           plutus-tx
-          plutus-tx-plugin
           plutus-use-cases
           plutus-ir
           plutus-playground/plutus-playground-server
           plutus-playground/plutus-playground-lib
 optimization: 2
 constraints: language-plutus-core +development
            , wallet-api +development
-           , plutus-tx-plugin +development
            , plutus-exe +development
            , plutus-core-interpreter +development
            , plutus-ir +development

default.nix

@@ -78,24 +78,28 @@ let
         filter = localLib.isPlutus;
       };
       customOverlays = optional forceError errorOverlay;
+      # Filter down to local packages, except those named in the given list
+      localButNot = nope: 
+        let okay = builtins.filter (name: !(builtins.elem name nope)) localLib.plutusPkgList;
+        in name: builtins.elem name okay;
     in self.callPackage localLib.iohkNix.haskellPackages {
       inherit forceDontCheck enableProfiling enablePhaseMetrics
       enableHaddockHydra enableBenchmarks fasterBuild enableDebugging
       enableSplitCheck customOverlays;
       pkgsGenerated = ./pkgs;
       filter = localLib.isPlutus;
       filterOverrides = {
-        splitCheck = let
-          dontSplit = [
+        splitCheck = localButNot [
             # Broken for things with test tool dependencies
             "wallet-api"
             "plutus-tx"
             # Broken for things which pick up other files at test runtime
             "plutus-playground-server"
           ];
-          # Split only local packages not in the don't split list
-          doSplit = builtins.filter (name: !(builtins.elem name dontSplit)) localLib.plutusPkgList;
-          in name: builtins.elem name doSplit;
+        haddock = localButNot [
+            # Haddock is broken for things with internal libraries
+            "plutus-tx"
+        ];
       };
       requiredOverlay = ./nix/overlays/required.nix;
     };

nix/overlays/required.nix

@@ -7,15 +7,20 @@ let
   addRealTimeTestLogs = drv: overrideCabal drv (attrs: {
     testTarget = "--show-details=streaming";
   });
+  doctest = opts: drv: overrideCabal drv (attrs: {
+    postCheck = "./Setup doctest --doctest-options=\"${opts}\"";
+  });
 in
 
 self: super: {
 
     ########################################################################
     # Overides of local packages
+    language-plutus-core = addRealTimeTestLogs super.language-plutus-core;
+    # cabal doctest doesn't seem to be clever enough to pick these up from the cabal file
+    plutus-tx = doctest "-pgmL markdown-unlit -XTemplateHaskell -XDeriveFunctor -XScopedTypeVariables" super.plutus-tx;
 
     ########################################################################
-    language-plutus-core = addRealTimeTestLogs super.language-plutus-core;
     # The base Haskell package builder
 
     mkDerivation = args: super.mkDerivation (args //

plutus-playground/plutus-playground-lib/plutus-playground-lib.cabal

@@ -40,7 +40,6 @@ library
     , bytestring
     , containers
     , plutus-tx -any
-    , plutus-tx-plugin -any
     , hint >= 0.9.0
     , http-media
     , insert-ordered-containers
@@ -79,4 +78,4 @@ test-suite playground-lib-test
   ghc-options:
     -Wall -Wnoncanonical-monad-instances
     -Wincomplete-uni-patterns -Wincomplete-record-updates
-    -Wredundant-constraints -Widentities
+    -Wredundant-constraints -Widentities