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/distribution-types/src/requirement.rs

@@ -205,4 +205,9 @@ impl RequirementSource {
             },
         }
     }
+
+    /// Returns `true` if the source is editable.
+    pub fn is_editable(&self) -> bool {
+        matches!(self, Self::Path { editable: true, .. })
+    }
 }

crates/distribution-types/src/resolution.rs

@@ -59,16 +59,9 @@ impl Resolution {
         self.0.is_empty()
     }
 
-    /// Return the set of [`Requirement`]s that this resolution represents, exclusive of any
-    /// editable requirements.
+    /// Return the set of [`Requirement`]s that this resolution represents.
     pub fn requirements(&self) -> Vec<Requirement> {
-        let mut requirements: Vec<_> = self
-            .0
-            .values()
-            // Remove editable requirements
-            .filter(|dist| !dist.is_editable())
-            .map(Requirement::from)
-            .collect();
+        let mut requirements: Vec<_> = self.0.values().map(Requirement::from).collect();
         requirements.sort_unstable_by(|a, b| a.name.cmp(&b.name));
         requirements
     }