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 can then organize
things a bit more neatly inside.

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

@@ -78,6 +78,10 @@ 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;
       # We can pass an evaluated version of our packages into
       # iohk-nix, and then we can also get out the compiler
       # so we make sure it uses the same one.
@@ -91,17 +95,17 @@ let
 
       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