Adds test coverage for #17991,
which includes some minor refactoring of the virtual environment test
infrastructure.

I tried to minimize stylistic changes, but there are still a few because
I was a little confused by the setup. I could see this evolving more in
the future, as I don't think the existing model can capture all the test
coverage I'm looking for.

…` or `VIRTUAL_ENV` (#18003)

Follow-up to #17991 ensuring we do
not allow detection of system environments when the origin is
`VIRTUAL_ENV` or a discovered `.venv` directory — i.e., those always
require a `pyvenv.cfg` file.

Following #17991, removes some of
#17222 which is no longer strictly
necessary. I don't actually think it's that ugly to have around? no
strong feelings on retaining it or not.

…#17991)

This adds basic support for non-virtual Python environments by accepting
a directory without a `pyvenv.cfg` which allows existing, subsequent
site-packages discovery logic to succeed. We can do better here in the
long-term, by adding more eager validation (for error messages) and
parsing the Python version from the discovered site-packages directory
(which isn't relevant yet, because we don't use the discovered Python
version from virtual environments as the default `--python-version` yet
either).

Related

- astral-sh/ty#265
- astral-sh/ty#193

You can review this commit by commit if it makes you happy.

I tested this manually; I think refactoring the test setup is going to
be a bit more invasive so I'll stack it on top (see
astral-sh#17996).

```
❯ uv run ty check --python /Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none/ -vv example
2025-05-09 12:06:33.685911 DEBUG Version: 0.0.0-alpha.7 (f9c4c8999 2025-05-08)
2025-05-09 12:06:33.685987 DEBUG Architecture: aarch64, OS: macos, case-sensitive: case-insensitive
2025-05-09 12:06:33.686002 DEBUG Searching for a project in '/Users/zb/workspace/ty'
2025-05-09 12:06:33.686123 DEBUG Resolving requires-python constraint: `>=3.8`
2025-05-09 12:06:33.686129 DEBUG Resolved requires-python constraint to: 3.8
2025-05-09 12:06:33.686142 DEBUG Project without `tool.ty` section: '/Users/zb/workspace/ty'
2025-05-09 12:06:33.686147 DEBUG Searching for a user-level configuration at `/Users/zb/.config/ty/ty.toml`
2025-05-09 12:06:33.686156 INFO Defaulting to python-platform `darwin`
2025-05-09 12:06:33.68636 INFO Python version: Python 3.8, platform: darwin
2025-05-09 12:06:33.686375 DEBUG Adding first-party search path '/Users/zb/workspace/ty'
2025-05-09 12:06:33.68638 DEBUG Using vendored stdlib
2025-05-09 12:06:33.686634 DEBUG Discovering site-packages paths from sys-prefix `/Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none` (`--python` argument')
2025-05-09 12:06:33.686667 DEBUG Attempting to parse virtual environment metadata at '/Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none/pyvenv.cfg'
2025-05-09 12:06:33.686671 DEBUG Searching for site-packages directory in `sys.prefix` path `/Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none`
2025-05-09 12:06:33.686702 DEBUG Resolved site-packages directories for this environment are: ["/Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none/lib/python3.10/site-packages"]
2025-05-09 12:06:33.686706 DEBUG Adding site-packages search path '/Users/zb/.local/share/uv/python/cpython-3.10.17-macos-aarch64-none/lib/python3.10/site-packages'
...

❯ uv run ty check --python /tmp -vv example
2025-05-09 15:36:10.819416 DEBUG Version: 0.0.0-alpha.7 (f9c4c8999 2025-05-08)
2025-05-09 15:36:10.819708 DEBUG Architecture: aarch64, OS: macos, case-sensitive: case-insensitive
2025-05-09 15:36:10.820118 DEBUG Searching for a project in '/Users/zb/workspace/ty'
2025-05-09 15:36:10.821652 DEBUG Resolving requires-python constraint: `>=3.8`
2025-05-09 15:36:10.821667 DEBUG Resolved requires-python constraint to: 3.8
2025-05-09 15:36:10.8217 DEBUG Project without `tool.ty` section: '/Users/zb/workspace/ty'
2025-05-09 15:36:10.821888 DEBUG Searching for a user-level configuration at `/Users/zb/.config/ty/ty.toml`
2025-05-09 15:36:10.822072 INFO Defaulting to python-platform `darwin`
2025-05-09 15:36:10.822439 INFO Python version: Python 3.8, platform: darwin
2025-05-09 15:36:10.822773 DEBUG Adding first-party search path '/Users/zb/workspace/ty'
2025-05-09 15:36:10.822929 DEBUG Using vendored stdlib
2025-05-09 15:36:10.829872 DEBUG Discovering site-packages paths from sys-prefix `/tmp` (`--python` argument')
2025-05-09 15:36:10.829911 DEBUG Attempting to parse virtual environment metadata at '/private/tmp/pyvenv.cfg'
2025-05-09 15:36:10.829917 DEBUG Searching for site-packages directory in `sys.prefix` path `/private/tmp`
ty failed
  Cause: Invalid search path settings
  Cause: Failed to discover the site-packages directory: Failed to search the `lib` directory of the Python installation at `sys.prefix` path `/private/tmp` for `site-packages`
```

…al-sh#17996)

Adds test coverage for astral-sh#17991,
which includes some minor refactoring of the virtual environment test
infrastructure.

I tried to minimize stylistic changes, but there are still a few because
I was a little confused by the setup. I could see this evolving more in
the future, as I don't think the existing model can capture all the test
coverage I'm looking for.

…` or `VIRTUAL_ENV` (astral-sh#18003)

Follow-up to astral-sh#17991 ensuring we do
not allow detection of system environments when the origin is
`VIRTUAL_ENV` or a discovered `.venv` directory — i.e., those always
require a `pyvenv.cfg` file.

Following astral-sh#17991, removes some of
astral-sh#17222 which is no longer strictly
necessary. I don't actually think it's that ugly to have around? no
strong feelings on retaining it or not.