tests/functional2: Fix Python LSP by adjusting imports

See: cl/4840

When importing Python modules, we include `functional2` in the module
path, like this:

    from functional2.testlib.fixtures.env import ManagedEnv

This means that python expects to see a file like
`functional2/testlib/fixtures/env.py`. We run `pytest` from `tests/` in
the `justfile` and have `tests/functional2/__init__.py` so `pytest` in
`meson` is able to find these imports.

However, language servers generally consider the `pyproject.toml` to be
the project root, so (e.g.) `pyright` is unable to follow any of the
`functional2` imports, leading to lots of spurious errors.

In cl/4840 I moved `tests/functional2/pyproject.toml` to
`tests/pyproject.toml`, which worked but was considered aesthetically
unappealing.

This diff is much larger but it's a more elegant solution.

Change-Id: I2983c7b87f88f59a4e3521451a9f5acd6a6a6964

tests/functional2/eval/test_eval_cli.py

@@ -3,8 +3,8 @@ from typing import Any
 
 import pytest
 
-from functional2.testlib.fixtures.file_helper import with_files, CopyFile, Symlink, File
-from functional2.testlib.fixtures.nix import Nix
+from testlib.fixtures.file_helper import with_files, CopyFile, Symlink, File
+from testlib.fixtures.nix import Nix
 
 
 @pytest.fixture(autouse=True)