Put functional tests in tests/functional

I think it is bad for these reasons when `tests/` contains a mix of
functional and integration tests

 - Concepts is harder to understand, the documentation makes a good
   unit vs functional vs integration distinction, but when the
   integration tests are just two subdirs within `tests/` this is not
   clear.

 - Source filtering in the `flake.nix` is more complex. We need to
   filter out some of the dirs from `tests/`, rather than simply pick
   the dirs we want and take all of them. This is a good sign the
   structure of what we are trying to do is not matching the structure
   of the files.

With this change we have a clean:
```shell-session
$ git show 'HEAD:tests'
tree HEAD:tests

functional/
installer/
nixos/
```

tests/functional/dependencies.nix

@@ -0,0 +1,48 @@
+{ hashInvalidator ? "" }:
+with import ./config.nix;
+
+let {
+
+  input0 = mkDerivation {
+    name = "dependencies-input-0";
+    buildCommand = "mkdir $out; echo foo > $out/bar";
+  };
+
+  input1 = mkDerivation {
+    name = "dependencies-input-1";
+    buildCommand = "mkdir $out; echo FOO > $out/foo";
+  };
+
+  input2 = mkDerivation {
+    name = "dependencies-input-2";
+    buildCommand = ''
+      mkdir $out
+      echo BAR > $out/bar
+      echo ${input0} > $out/input0
+    '';
+  };
+
+  fod_input = mkDerivation {
+    name = "fod-input";
+    buildCommand = ''
+      echo ${hashInvalidator}
+      echo FOD > $out
+    '';
+    outputHashMode = "flat";
+    outputHashAlgo = "sha256";
+    outputHash = "1dq9p0hnm1y75q2x40fws5887bq1r840hzdxak0a9djbwvx0b16d";
+  };
+
+  body = mkDerivation {
+    name = "dependencies-top";
+    builder = ./dependencies.builder0.sh + "/FOOBAR/../.";
+    input1 = input1 + "/.";
+    input2 = "${input2}/.";
+    input1_drv = input1;
+    input2_drv = input2;
+    input0_drv = input0;
+    fod_input_drv = fod_input;
+    meta.description = "Random test package";
+  };
+
+}