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
