Run interpreter discovery under -I mode (#2552)

## Summary

If you have a file `typing.py` in the current working directory, `python
-m` doesn't work in some Python versions:

```sh
❯ python -m foo
Could not import runpy module
Traceback (most recent call last):
  File "/Users/crmarsh/.local/share/rtx/installs/python/3.9.18/lib/python3.9/runpy.py", line 15, in <module>
    import importlib.util
  File "/Users/crmarsh/.local/share/rtx/installs/python/3.9.18/lib/python3.9/importlib/util.py", line 2, in <module>
    from . import abc
  File "/Users/crmarsh/.local/share/rtx/installs/python/3.9.18/lib/python3.9/importlib/abc.py", line 17, in <module>
    from typing import Protocol, runtime_checkable
ImportError: cannot import name 'Protocol' from 'typing' (/Users/crmarsh/workspace/uv/typing.py)
```

This did _not_ cause problems for us on Python 3.11 or later, because we
set `PYTHONSAFEPATH`, which avoids adding the current working directory
to `sys.path`. However, on earlier versions, we _were_ failing with the
above. (It's important that we run interpreter discovery in the current
working directory, since doing otherwise breaks pyenv shims.)

The fix implemented here uses `-I` to run Python in isolated mode, which
is even stricter. The downside of isolated mode is that we currently
rely on setting `PYTHONPATH` to find the "fake module" that we create on
disk, and `-I` means `PYTHONPATH` is totally ignored. So, instead, we
run a script directly, and that _script_ injects the path we care about
into `PYTHONSAFEPATH`.

Closes https://github.com/astral-sh/uv/issues/2547.

crates/uv-interpreter/src/interpreter.rs

@@ -15,7 +15,7 @@ use platform_tags::Platform;
 use platform_tags::{Tags, TagsError};
 use pypi_types::Scheme;
 use uv_cache::{Cache, CacheBucket, CachedByTimestamp, Freshness, Timestamp};
-use uv_fs::{write_atomic_sync, Simplified};
+use uv_fs::{write_atomic_sync, PythonExt, Simplified};
 
 use crate::Error;
 use crate::Virtualenv;
@@ -369,11 +369,17 @@ impl InterpreterInfo {
         let tempdir = tempfile::tempdir_in(cache.root())?;
         Self::setup_python_query_files(tempdir.path())?;
 
+        // Sanitize the path by (1) running under isolated mode (`-I`) to ignore any site packages
+        // modifications, and then (2) adding the path containing our query script to the front of
+        // `sys.path` so that we can import it.
+        let script = format!(
+            r#"import sys; sys.path = ["{}"] + sys.path; from python.get_interpreter_info import main; main()"#,
+            tempdir.path().escape_for_python()
+        );
         let output = Command::new(interpreter)
-            .arg("-m")
-            .arg("python.get_interpreter_info")
-            .env("PYTHONPATH", tempdir.path())
-            .env("PYTHONSAFEPATH", "1")
+            .arg("-I")
+            .arg("-c")
+            .arg(script)
             .output()
             .map_err(|err| Error::PythonSubcommandLaunch {
                 interpreter: interpreter.to_path_buf(),