Port Pyright's import resolver to Rust (#5381)

## Summary

This PR contains the first step towards enabling robust first-party,
third-party, and standard library import resolution in Ruff (including
support for `typeshed`, stub files, native modules, etc.) by porting
Pyright's import resolver to Rust.

The strategy taken here was to start with a more-or-less direct port of
the Pyright's TypeScript resolver. The code is intentionally similar,
and the test suite is effectively a superset of Pyright's test suite for
its own resolver. Due to the nature of the port, the code is very, very
non-idiomatic for Rust. The code is also entirely unused outside of the
test suite, and no effort has been made to integrate it with the rest of
the codebase.

Future work will include:

- Refactoring the code (now that it works) to match Rust and Ruff
idioms.
- Further testing, in practice, to ensure that the resolver can resolve
imports in a complex project, when provided with a virtual environment
path.
- Caching, to minimize filesystem lookups and redundant resolutions.
- Integration into Ruff itself (use Ruff's existing settings, find rules
that can make use of robust resolution, etc.)

crates/ruff_python_resolver/src/config.rs

@@ -0,0 +1,26 @@
+use std::path::PathBuf;
+
+use crate::python_version::PythonVersion;
+
+pub(crate) struct Config {
+    /// Path to python interpreter.
+    pub(crate) python_path: Option<PathBuf>,
+
+    /// Path to use for typeshed definitions.
+    pub(crate) typeshed_path: Option<PathBuf>,
+
+    /// Path to custom typings (stub) modules.
+    pub(crate) stub_path: Option<PathBuf>,
+
+    /// Path to a directory containing one or more virtual environment
+    /// directories. This is used in conjunction with the "venv" name in
+    /// the config file to identify the python environment used for resolving
+    /// third-party modules.
+    pub(crate) venv_path: Option<PathBuf>,
+
+    /// Default venv environment.
+    pub(crate) venv: Option<PathBuf>,
+
+    /// Default Python version. Can be overridden by ExecutionEnvironment.
+    pub(crate) default_python_version: Option<PythonVersion>,
+}