Make hashes optional (#910)

There is no guarantee that indexes provide hashes at all or the sha256
we support specifically. [PEP
503](https://peps.python.org/pep-0503/#specification):

> The URL SHOULD include a hash in the form of a URL fragment with the
following syntax: #<hashname>=<hashvalue>, where <hashname> is the
lowercase name of the hash function (such as sha256) and <hashvalue> is
the hex encoded digest.

We instead use the url as input to generate a hash when caching.

crates/puffin-resolver/src/resolution.rs

@@ -270,8 +270,10 @@ impl std::fmt::Display for DisplayResolutionGraph<'_> {
                     .filter(|hashes| !hashes.is_empty())
                 {
                     for hash in hashes {
-                        writeln!(f, " \\")?;
-                        write!(f, "    --hash={hash}")?;
+                        if let Some(hash) = hash.to_string() {
+                            writeln!(f, " \\")?;
+                            write!(f, "    --hash={hash}")?;
+                        }
                     }
                 }
             }