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.

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 //