Rename and document venv discoveries (#2334)

Preparing for #2058, i found it hard to follow where which discovery
function gets called. I moved all the discovery functions to a
`find_python` module (some exposed through `PythonEnvironment`) and
documented which subcommand uses which python discovery strategy.

No functional changes.

![new uv-virtualenv docs
page](https://github.com/astral-sh/uv/assets/6826232/cd56df8a-754d-4640-9e7a-e1f9baf6441c)

crates/uv-interpreter/src/python_environment.rs

@@ -10,7 +10,7 @@ use uv_fs::{LockedFile, Simplified};
 use crate::cfg::PyVenvConfiguration;
 use crate::{find_default_python, find_requested_python, Error, Interpreter};
 
-/// A Python environment, consisting of a Python [`Interpreter`] and a root directory.
+/// A Python environment, consisting of a Python [`Interpreter`] and its associated paths.
 #[derive(Debug, Clone)]
 pub struct PythonEnvironment {
     root: PathBuf,