Build source distributions in the resolver (#138)

This is isn't ready, but it can resolve
`meine_stadt_transparent==0.2.14`.

The source distributions are currently being built serially one after
the other, i don't know if that is incidentally due to the resolution
order, because sdist building is blocking or because of something in the
resolver that could be improved.

It's a bit annoying that the thing that was supposed to do http requests
now suddenly also has to a whole download/unpack/resolve/install/build
routine, it messes up the type hierarchy. The much bigger problem though
is avoid recursive crate dependencies, it's the reason for the callback
and for splitting the builder into two crates (badly named atm)

crates/pep508-rs/src/marker.rs

@@ -312,6 +312,12 @@ impl FromStr for StringVersion {
     }
 }
 
+impl Display for StringVersion {
+    fn fmt(&self, f: &mut Formatter<'_>) -> std::fmt::Result {
+        self.version.fmt(f)
+    }
+}
+
 #[cfg(feature = "serde")]
 impl Serialize for StringVersion {
     fn serialize<S>(&self, serializer: S) -> Result<S::Ok, S::Error>