This PR ensures that we always put `./src` before `.` in our list of
first-party search paths. This better emulates the fact that at runtime,
the module name of a file `src/foo.py` would almost certainly be `foo`
rather than `src.foo`.
I wondered if fixing this might fix
https://github.com/astral-sh/ruff/pull/20603#issuecomment-3345317444. It
seems like that's not the case, but it also seems like it leads to
better diagnostics because we report much more intuitive module names to
the user in our error messages -- so, it's probably a good change
anyway.
## Summary
I felt it was safer to add the `python` folder *in addition* to a
possibly-existing `src` folder, even though the `src` folder only
contains Rust code for `maturin`-based projects. There might be
non-maturin projects where a `python` folder exists for other reasons,
next to a normal `src` layout.
closes https://github.com/astral-sh/ty/issues/1120
## Test Plan
Tested locally on the egglog-python project.
This is a port of the logic in https://github.com/astral-sh/uv/pull/7691
The basic idea is we use CONDA_DEFAULT_ENV as a signal for whether
CONDA_PREFIX is just the ambient system conda install, or the user has
explicitly activated a custom one. If the former, then the conda is
treated like a system install (having lowest priority). If the latter,
the conda is treated like an activated venv (having priority over
everything but an Actual activated venv).
Fixes https://github.com/astral-sh/ty/issues/611
## Summary
Fixes https://github.com/astral-sh/ty/issues/640. If a user passes
`--python=<some-virtual-environment>/bin/python`, we must avoid
canonicalizing the path until we've traversed upwards to find the
`sys.prefix` directory (`<some-virtual-environment>`). On Unix systems,
`<sys.prefix>/bin/python` is often a symlink to a system interpreter; if
we resolve the symlink too easily then we'll add the system
interpreter's `site-packages` directory as a search path rather than the
virtual environment's directory.
## Test Plan
I added an integration test to
`crates/ty/tests/cli/python_environment.rs` which fails on `main`. I
also manually tested locally that running `cargo run -p ty check foo.py
--python=.venv/bin/python -vv` now prints this log to the terminal
```
2025-06-20 18:35:24.57702 DEBUG Resolved site-packages directories for this virtual environment are: SitePackagesPaths({"/Users/alexw/dev/ruff/.venv/lib/python3.13/site-packages"})
```
Whereas it previously resolved `site-packages` to my system
intallation's `site-packages` directory
