agda: Wrapper is no longer built by default

Instead it is provided to the user who can choose whether or not
to include it in the final derivati. Example of including would
be:

```nix
callPackage ... (self: { inherit (self.extras) extraThing; })
```

These extras are also available downstream without being built by
default. This is achieved with `passthru`.

pkgs/build-support/agda/default.nix

@@ -22,6 +22,9 @@ in
         propagatedBuildInputs = filter (y : ! (y == null)) x.propagatedBuildInputs;
         propagatedUserEnvPkgs = filter (y : ! (y == null)) x.propagatedUserEnvPkgs;
         everythingFile = if x.everythingFile == "" then "Everything.agda" else x.everythingFile;
+
+        passthru = { inherit (x) extras; };
+        extras = null;
       };
 
       defaults = self : {
@@ -63,25 +66,11 @@ in
                       ++ [ "." ];
         buildFlags = unwords (map (x: "-i " + x) self.includeDirs);
 
-        # We expose this as a mere convenience for any tools.
-        AGDA_PACKAGE_PATH = concatMapStrings (x: x + ":") self.buildDependsAgdaShareAgda;
-
-        # Makes a wrapper available to the user. Very useful in
-        # nix-shell where all dependencies are -i'd.
-        agdaWrapper = writeScriptBin "agda" ''
-          ${Agda}/bin/agda ${self.buildFlags} "$@"
-        '';
-
-        # configurePhase is idempotent
-        configurePhase = ''
-          runHook preConfigure
-          export PATH="${self.agdaWrapper}/bin:$PATH"
-          runHook postConfigure
-        '';
+        agdaWithArgs = "${Agda}/bin/agda ${self.buildFlags}";
 
         buildPhase = ''
           runHook preBuild
-          ${Agda}/bin/agda ${self.buildFlags} ${self.everythingFile}
+          ${self.agdaWithArgs} ${self.everythingFile}
           runHook postBuild
         '';
 
@@ -91,6 +80,24 @@ in
           cp -pR ${unwords self.sourceDirectories} ${mapInside self.topSourceDirectories} $out/share/agda
           runHook postInstall
         '';
+
+        # Optionally-built conveniences
+        extras = {
+          # Makes a wrapper available to the user. Very useful in
+          # nix-shell where all dependencies are -i'd.
+          agdaWrapper = writeScriptBin "agda" ''
+            ${self.agdaWithArgs} "$@"
+          '';
+
+          # Use this to stick `agdaWrapper` at the front of the PATH:
+          #
+          # agda.mkDerivation (self: { PATH = self.extras.agdaWrapperPATH; })
+          #
+          # Not sure this is the best way to handle conflicts....
+          agdaWrapperPATH = "${self.extras.agdaWrapper}/bin:$PATH";
+
+          AGDA_PACKAGE_PATH = concatMapStrings (x: x + ":") self.buildDependsAgdaShareAgda;
+        };
       };
     in stdenv.mkDerivation
          (postprocess (let super = defaults self // args self;