Use the canonical ImplementationName -> &str implementation (#14337)

Motivated by some code duplication highlighted in
https://github.com/astral-sh/uv/pull/14201, I noticed we weren't taking
advantage of the existing implementation for casting to a str here.
Unfortunately, we do need a special case for CPython still.

crates/uv-python/src/implementation.rs

@@ -44,6 +44,13 @@ impl ImplementationName {
             Self::GraalPy => "GraalPy",
         }
     }
+
+    pub fn executable_name(self) -> &'static str {
+        match self {
+            Self::CPython => "python",
+            Self::PyPy | Self::GraalPy => self.into(),
+        }
+    }
 }
 
 impl LenientImplementationName {
@@ -53,6 +60,13 @@ impl LenientImplementationName {
             Self::Unknown(name) => name,
         }
     }
+
+    pub fn executable_name(&self) -> &str {
+        match self {
+            Self::Known(implementation) => implementation.executable_name(),
+            Self::Unknown(name) => name,
+        }
+    }
 }
 
 impl From<&ImplementationName> for &'static str {