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/flakes/follow-paths.sh

@@ -0,0 +1,232 @@
+source ./common.sh
+
+requireGit
+
+flakeFollowsA=$TEST_ROOT/follows/flakeA
+flakeFollowsB=$TEST_ROOT/follows/flakeA/flakeB
+flakeFollowsC=$TEST_ROOT/follows/flakeA/flakeB/flakeC
+flakeFollowsD=$TEST_ROOT/follows/flakeA/flakeD
+flakeFollowsE=$TEST_ROOT/follows/flakeA/flakeE
+
+# Test following path flakerefs.
+createGitRepo $flakeFollowsA
+mkdir -p $flakeFollowsB
+mkdir -p $flakeFollowsC
+mkdir -p $flakeFollowsD
+mkdir -p $flakeFollowsE
+
+cat > $flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+            inputs.foobar.follows = "foobar";
+        };
+
+        foobar.url = "path:$flakeFollowsA/flakeE";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+cat > $flakeFollowsB/flake.nix <<EOF
+{
+    description = "Flake B";
+    inputs = {
+        foobar.url = "path:$flakeFollowsA/flakeE";
+        goodoo.follows = "C/goodoo";
+        C = {
+            url = "path:./flakeC";
+            inputs.foobar.follows = "foobar";
+        };
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+cat > $flakeFollowsC/flake.nix <<EOF
+{
+    description = "Flake C";
+    inputs = {
+        foobar.url = "path:$flakeFollowsA/flakeE";
+        goodoo.follows = "foobar";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+cat > $flakeFollowsD/flake.nix <<EOF
+{
+    description = "Flake D";
+    inputs = {};
+    outputs = { ... }: {};
+}
+EOF
+
+cat > $flakeFollowsE/flake.nix <<EOF
+{
+    description = "Flake E";
+    inputs = {};
+    outputs = { ... }: {};
+}
+EOF
+
+git -C $flakeFollowsA add flake.nix flakeB/flake.nix \
+  flakeB/flakeC/flake.nix flakeD/flake.nix flakeE/flake.nix
+
+nix flake metadata $flakeFollowsA
+
+nix flake update $flakeFollowsA
+
+nix flake lock $flakeFollowsA
+
+oldLock="$(cat "$flakeFollowsA/flake.lock")"
+
+# Ensure that locking twice doesn't change anything
+
+nix flake lock $flakeFollowsA
+
+newLock="$(cat "$flakeFollowsA/flake.lock")"
+
+diff <(echo "$newLock") <(echo "$oldLock")
+
+[[ $(jq -c .nodes.B.inputs.C $flakeFollowsA/flake.lock) = '"C"' ]]
+[[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '["foobar"]' ]]
+[[ $(jq -c .nodes.C.inputs.foobar $flakeFollowsA/flake.lock) = '["B","foobar"]' ]]
+
+# Ensure removing follows from flake.nix removes them from the lockfile
+
+cat > $flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+        };
+        D.url = "path:./flakeD";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+nix flake lock $flakeFollowsA
+
+[[ $(jq -c .nodes.B.inputs.foobar $flakeFollowsA/flake.lock) = '"foobar"' ]]
+jq -r -c '.nodes | keys | .[]' $flakeFollowsA/flake.lock | grep "^foobar$"
+
+# Ensure a relative path is not allowed to go outside the store path
+cat > $flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B.url = "path:../flakeB";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+git -C $flakeFollowsA add flake.nix
+
+expect 1 nix flake lock $flakeFollowsA 2>&1 | grep 'points outside'
+
+# Non-existant follows should print a warning.
+cat >$flakeFollowsA/flake.nix <<EOF
+{
+    description = "Flake A";
+    inputs.B = {
+        url = "path:./flakeB";
+        inputs.invalid.follows = "D";
+        inputs.invalid2.url = "path:./flakeD";
+    };
+    inputs.D.url = "path:./flakeD";
+    outputs = { ... }: {};
+}
+EOF
+
+git -C $flakeFollowsA add flake.nix
+
+nix flake lock "$flakeFollowsA" 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid'"
+nix flake lock "$flakeFollowsA" 2>&1 | grep "warning: input 'B' has an override for a non-existent input 'invalid2'"
+
+# Now test follow path overloading
+# This tests a lockfile checking regression https://github.com/NixOS/nix/pull/8819
+#
+# We construct the following graph, where p->q means p has input q.
+# A double edge means that the edge gets overridden using `follows`.
+#
+#      A
+#     / \
+#    /   \
+#   v     v
+#   B ==> C   --- follows declared in A
+#    \\  /
+#     \\/     --- follows declared in B
+#      v
+#      D
+#
+# The message was
+#    error: input 'B/D' follows a non-existent input 'B/C/D'
+# 
+# Note that for `B` to resolve its follow for `D`, it needs `C/D`, for which it needs to resolve the follow on `C` first.
+flakeFollowsOverloadA="$TEST_ROOT/follows/overload/flakeA"
+flakeFollowsOverloadB="$TEST_ROOT/follows/overload/flakeA/flakeB"
+flakeFollowsOverloadC="$TEST_ROOT/follows/overload/flakeA/flakeB/flakeC"
+flakeFollowsOverloadD="$TEST_ROOT/follows/overload/flakeA/flakeB/flakeC/flakeD"
+
+# Test following path flakerefs.
+createGitRepo "$flakeFollowsOverloadA"
+mkdir -p "$flakeFollowsOverloadB"
+mkdir -p "$flakeFollowsOverloadC"
+mkdir -p "$flakeFollowsOverloadD"
+
+cat > "$flakeFollowsOverloadD/flake.nix" <<EOF
+{
+    description = "Flake D";
+    inputs = {};
+    outputs = { ... }: {};
+}
+EOF
+
+cat > "$flakeFollowsOverloadC/flake.nix" <<EOF
+{
+    description = "Flake C";
+    inputs.D.url = "path:./flakeD";
+    outputs = { ... }: {};
+}
+EOF
+
+cat > "$flakeFollowsOverloadB/flake.nix" <<EOF
+{
+    description = "Flake B";
+    inputs = {
+        C = {
+            url = "path:./flakeC";
+        };
+        D.follows = "C/D";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+# input B/D should be able to be found...
+cat > "$flakeFollowsOverloadA/flake.nix" <<EOF
+{
+    description = "Flake A";
+    inputs = {
+        B = {
+            url = "path:./flakeB";
+            inputs.C.follows = "C";
+        };
+        C.url = "path:./flakeB/flakeC";
+    };
+    outputs = { ... }: {};
+}
+EOF
+
+git -C "$flakeFollowsOverloadA" add flake.nix flakeB/flake.nix \
+  flakeB/flakeC/flake.nix flakeB/flakeC/flakeD/flake.nix
+
+nix flake metadata "$flakeFollowsOverloadA"
+nix flake update "$flakeFollowsOverloadA"
+nix flake lock "$flakeFollowsOverloadA"