Use common routines for pip install and pip sync (#3737)

## Summary

This PR takes the functions used in `pip install`, moves them into a
common module, and then replaces all the `pip sync` logic with calls
into those functions. The net effect is that `pip install` and `pip
sync` share far more code and demonstrate much more consistent behavior.

Closes https://github.com/astral-sh/uv/issues/3555.

crates/uv-configuration/src/package_options.rs

@@ -44,9 +44,10 @@ impl Reinstall {
 }
 
 /// Whether to allow package upgrades.
-#[derive(Debug, Clone)]
+#[derive(Debug, Default, Clone)]
 pub enum Upgrade {
     /// Prefer pinned versions from the existing lockfile, if possible.
+    #[default]
     None,
 
     /// Allow package upgrades for all packages, ignoring the existing lockfile.