[ty] Improve tests for site-packages discovery (#18374)

## Summary

- Convert tests demonstrating our resilience to malformed/absent
`version` fields in `pyvenf.cfg` files to mdtests. Also make them more
expansive.
- Convert the regression test I added in
https://github.com/astral-sh/ruff/pull/18157 to an mdtest
- Add comments next to unit tests that cannot be converted to mdtests
(but where it's not obvious why they can't) so I don't have to do this
exercise again 😄
- In `site_packages.rs`, factor out the logic for figuring out where we
expect the system-installation `site-packages` to be. Currently we have
the same logic twice.

## Test Plan

`cargo test -p ty_python_semantic`

crates/ty_python_semantic/resources/mdtest/import/site_packages_discovery.md

@@ -1,5 +1,115 @@
 # Tests for `site-packages` discovery
 
+## Malformed or absent `version` fields
+
+The `version`/`version_info` key in a `pyvenv.cfg` file is provided by most virtual-environment
+creation tools to indicate the Python version the virtual environment is for. They key is useful for
+our purposes, so we try to parse it when possible. However, the key is not read by the CPython
+standard library, and is provided under different keys depending on which virtual-environment
+creation tool created the `pyvenv.cfg` file (the stdlib `venv` module calls the key `version`,
+whereas uv and virtualenv both call it `version_info`). We therefore do not return an error when
+discovering a virtual environment's `site-packages` directory if the virtula environment contains a
+`pyvenv.cfg` file which doesn't have this key, or if the associated value of the key doesn't parse
+according to our expectations. The file isn't really *invalid* in this situation.
+
+### No `version` field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
+### Malformed stdlib-style version field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version = wut
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
+### Malformed uv-style version field
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version_info = no-really-wut
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```
+
 ## Ephemeral uv environments
 
 If you use the `--with` flag when invoking `uv run`, uv will create an "ephemeral" virtual
@@ -57,3 +167,41 @@ from bar import Y
 reveal_type(X)  # revealed: int
 reveal_type(Y)  # revealed: str
 ```
+
+## `pyvenv.cfg` files with unusual values
+
+`pyvenv.cfg` files can have unusual values in them, which can contain arbitrary characters. This
+includes `=` characters. The following is a regression test for
+<https://github.com/astral-sh/ty/issues/430>.
+
+```toml
+[environment]
+python = "/.venv"
+```
+
+`/.venv/pyvenv.cfg`:
+
+```cfg
+home = /doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin
+version_info = 3.13
+command = /.pyenv/versions/3.13.3/bin/python3.13 -m venv --without-pip --prompt="python-default/3.13.3" /somewhere-else/python/virtualenvs/python-default/3.13.3
+```
+
+`/doo/doo/wop/cpython-3.13.2-macos-aarch64-none/bin/python`:
+
+```text
+```
+
+`/.venv/<path-to-site-packages>/foo.py`:
+
+```py
+X: int = 42
+```
+
+`/src/main.py`:
+
+```py
+from foo import X
+
+reveal_type(X)  # revealed: int
+```