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/
```

(cherry picked from commit 68c81c7375)

tests/functional/flakes/inputs.sh

@@ -0,0 +1,80 @@
+source ./common.sh
+
+requireGit
+
+
+test_subdir_self_path() {
+    baseDir=$TEST_ROOT/$RANDOM
+    flakeDir=$baseDir/b-low
+    mkdir -p $flakeDir
+    writeSimpleFlake $baseDir
+    writeSimpleFlake $flakeDir
+
+    echo all good > $flakeDir/message
+    cat > $flakeDir/flake.nix <<EOF
+{
+  outputs = inputs: rec {
+    packages.$system = rec {
+      default =
+        assert builtins.readFile ./message == "all good\n";
+        assert builtins.readFile (inputs.self + "/message") == "all good\n";
+        import ./simple.nix;
+    };
+  };
+}
+EOF
+    (
+        nix build $baseDir?dir=b-low --no-link
+    )
+}
+test_subdir_self_path
+
+
+test_git_subdir_self_path() {
+    repoDir=$TEST_ROOT/repo-$RANDOM
+    createGitRepo $repoDir
+    flakeDir=$repoDir/b-low
+    mkdir -p $flakeDir
+    writeSimpleFlake $repoDir
+    writeSimpleFlake $flakeDir
+
+    echo all good > $flakeDir/message
+    cat > $flakeDir/flake.nix <<EOF
+{
+  outputs = inputs: rec {
+    packages.$system = rec {
+      default =
+        assert builtins.readFile ./message == "all good\n";
+        assert builtins.readFile (inputs.self + "/message") == "all good\n";
+        assert inputs.self.outPath == inputs.self.sourceInfo.outPath + "/b-low";
+        import ./simple.nix;
+    };
+  };
+}
+EOF
+    (
+        cd $flakeDir
+        git add .
+        git commit -m init
+        # nix build
+    )
+
+    clientDir=$TEST_ROOT/client-$RANDOM
+    mkdir -p $clientDir
+    cat > $clientDir/flake.nix <<EOF
+{
+  inputs.inp = {
+    type = "git";
+    url = "file://$repoDir";
+    dir = "b-low";
+  };
+
+  outputs = inputs: rec {
+    packages = inputs.inp.packages;
+  };
+}
+EOF
+    nix build $clientDir --no-link
+
+}
+test_git_subdir_self_path