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/fetchTree-file.sh

@@ -0,0 +1,117 @@
+source common.sh
+
+clearStore
+
+cd "$TEST_ROOT"
+
+test_fetch_file () {
+    echo foo > test_input
+
+    input_hash="$(nix hash path test_input)"
+
+    nix eval --impure --file - <<EOF
+    let
+        tree = builtins.fetchTree { type = "file"; url = "file://$PWD/test_input"; };
+    in
+    assert (tree.narHash == "$input_hash");
+    tree
+EOF
+}
+
+# Make sure that `http(s)` and `file` flake inputs are properly extracted when
+# they should be, and treated as opaque files when they should be
+test_file_flake_input () {
+    rm -fr "$TEST_ROOT/testFlake";
+    mkdir "$TEST_ROOT/testFlake";
+    pushd testFlake
+
+    mkdir inputs
+    echo foo > inputs/test_input_file
+    echo '{ outputs = { self }: { }; }' > inputs/flake.nix
+    tar cfa test_input.tar.gz inputs
+    cp test_input.tar.gz test_input_no_ext
+    input_tarball_hash="$(nix hash path test_input.tar.gz)"
+    input_directory_hash="$(nix hash path inputs)"
+
+    cat <<EOF > flake.nix
+    {
+        inputs.no_ext_default_no_unpack = {
+            url = "file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.no_ext_explicit_unpack = {
+            url = "tarball+file://$PWD/test_input_no_ext";
+            flake = false;
+        };
+        inputs.tarball_default_unpack = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        inputs.tarball_explicit_no_unpack = {
+            url = "file+file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        inputs.flake_no_ext = {
+            url = "file://$PWD/test_input_no_ext";
+        };
+        outputs = { ... }: {};
+    }
+EOF
+
+    nix flake update
+    nix eval --file - <<EOF
+    with (builtins.fromJSON (builtins.readFile ./flake.lock));
+
+    # Non-flake inputs whose extension doesn’t match a known archive format should
+    # not be unpacked by default
+    assert (nodes.no_ext_default_no_unpack.locked.type == "file");
+    assert (nodes.no_ext_default_no_unpack.locked.unpack or false == false);
+    assert (nodes.no_ext_default_no_unpack.locked.narHash == "$input_tarball_hash");
+
+    # For backwards compatibility, flake inputs that correspond to the
+    # old 'tarball' fetcher should still have their type set to 'tarball'
+    assert (nodes.tarball_default_unpack.locked.type == "tarball");
+    # Unless explicitely specified, the 'unpack' parameter shouldn’t appear here
+    # because that would break older Nix versions
+    assert (!nodes.tarball_default_unpack.locked ? unpack);
+    assert (nodes.tarball_default_unpack.locked.narHash == "$input_directory_hash");
+
+    # Explicitely passing the unpack parameter should enforce the desired behavior
+    assert (nodes.no_ext_explicit_unpack.locked.narHash == nodes.tarball_default_unpack.locked.narHash);
+    assert (nodes.tarball_explicit_no_unpack.locked.narHash == nodes.no_ext_default_no_unpack.locked.narHash);
+
+    # Flake inputs should always be tarballs
+    assert (nodes.flake_no_ext.locked.type == "tarball");
+
+    true
+EOF
+
+    # Test tarball URLs on the command line.
+    [[ $(nix flake metadata --json file://$PWD/test_input_no_ext | jq -r .resolved.type) = tarball ]]
+
+    popd
+
+    [[ -z "${NIX_DAEMON_PACKAGE-}" ]] && return 0
+
+    # Ensure that a lockfile generated by the current Nix for tarball inputs
+    # can still be read by an older Nix
+
+    cat <<EOF > flake.nix
+    {
+        inputs.tarball = {
+            url = "file://$PWD/test_input.tar.gz";
+            flake = false;
+        };
+        outputs = { self, tarball }: {
+            foo = builtins.readFile "\${tarball}/test_input_file";
+        };
+    }
+    nix flake update
+
+    clearStore
+    "$NIX_DAEMON_PACKAGE/bin/nix" eval .#foo
+EOF
+}
+
+test_fetch_file
+test_file_flake_input