Problem: Without fractalide in the context, requires don't work

Solution: Use extraSrcs to put compiler-lib into the fractalide
derivation.

racket -l- raco test works, as long as you don't try to run it inside
a Nix derivation. If you try to do it inside Nix, raco tries and fails
to create $HOME.

But it's slower than a full racket plus a user-installed fractalide:

```
$ time raco test modules/rkt/rkt-fbp/agents/plumbing/
[ . . . ]
raco test: (submod "modules/rkt/rkt-fbp/agents/plumbing/transform-in-msgs.rkt" test)
raco test: (submod "modules/rkt/rkt-fbp/agents/plumbing/transform-ins-msgs.rkt" test)
18 tests passed

real	0m13,330s
user	0m11,797s
sys	0m1,541s
```

```
pkgs$ time $(nix-build -A fractalide-tests-pkg.env)/bin/racket -l- raco test $(nix-build -A fractalide-tests-pkg.env)/share/racket/pkgs/*/modules/rkt/rkt-fbp/agents/plumbing
[ . . . ]
raco test: (submod "/nix/store/0siifhd8zjlkq2j793la97p0r19j3in8-fractalide-env/share/racket/pkgs/fractalide/modules/rkt/rkt-fbp/agents/plumbing/transform-in-msgs.rkt" test)
raco test: @(test-responsible '("[email protected]" "[email protected]"))
raco test: (submod "/nix/store/0siifhd8zjlkq2j793la97p0r19j3in8-fractalide-env/share/racket/pkgs/fractalide/modules/rkt/rkt-fbp/agents/plumbing/transform-ins-msgs.rkt" test)
raco test: @(test-responsible '("[email protected]" "[email protected]"))
18 tests passed

real	0m29,413s
user	0m15,441s
sys	0m12,025s
```

pkgs/default.nix

@@ -46,21 +46,23 @@ pkgs {
       };
 
       # fractalide/racket2nix#78 workaround
-      # This simple switcheroo only works because fractalide happens to depend on all of
+      # I considered just flattening, but fractalide/racket2nix#156
+
+      # This simple addition works because fractalide happens to depend on all of
       # compiler-lib's dependencies (because it happens to depend on compiler-lib).
       fractalide-tests-pkg = fractalide.overrideRacketDerivation (oldAttrs: {
-        src = fetchurl {
+        extraSrcs = [(fetchurl {
           url = "https://download.racket-lang.org/releases/6.12/pkgs/compiler-lib.zip";
           sha1 = "8921c26c498e920aca398df7afb0ab486636430f";
-        };
+        })];
         # Remove compiler-lib from its own dependencies.
         racketBuildInputs = builtins.filter (input: input.name != "compiler-lib") oldAttrs.racketBuildInputs;
       });
 
       fractalide-tests = self.runCommand "fractalide-tests" {
         buildInputs = [ fractalide-tests-pkg.env ];
       } ''
-        racket -l- raco test ${fractalide.env}/share/racket/pkgs/*/modules/rkt/rkt-fbp/agents
+        racket -l- raco test ${fractalide-tests-pkg.env}/share/racket/pkgs/*/modules/rkt/rkt-fbp/agents
       '';
     })
   ];