Improve PEP 691 compatibility (#428)

[PEP 691](https://peps.python.org/pep-0691/#project-detail) has slightly
different, more relaxed rules around file metadata. These changes are
now reflected in the `File` struct. This will make it easier to support
alternative indices.

I had expected that i need to introduce a separate type for that, so i'm
happy it's two `Option`s more and an alias.

Part of #412

crates/puffin-distribution/src/lib.rs

@@ -263,7 +263,7 @@ impl RemoteSource for RegistryBuiltDist {
     }
 
     fn size(&self) -> Option<usize> {
-        Some(self.file.size)
+        self.file.size
     }
 }
 
@@ -273,7 +273,7 @@ impl RemoteSource for RegistrySourceDist {
     }
 
     fn size(&self) -> Option<usize> {
-        Some(self.file.size)
+        self.file.size
     }
 }