Move conflicting dependencies into PubGrub (#1796)

## Summary

This revives a PR from long ago
(https://github.com/astral-sh/uv/pull/383 and
https://github.com/zanieb/pubgrub/pull/24) that modifies how we deal
with dependencies that are declared multiple times within a single
package.

To quote from the originating PR:

> Uses an experimental pubgrub branch (#370) that allows us to handle
multiple version ranges for a single dependency to the solver which
results in better error messages because the derivation tree contains
all of the relevant versions. Previously, the version ranges were merged
(by us) in the resolver before handing them to pubgrub since only one
range could be provided per package. Since we don't merge the versions
anymore, we no longer give the solver an empty range for conflicting
requirements; instead the solver comes to that conclusion from the
provided versions. You can see the improved error message for direct
dependencies in [this
snapshot](https://github.com/astral-sh/puffin/pull/383/files#diff-a0437f2c20cde5e2f15199a3bf81a102b92580063268417847ec9c793a115bd0).

The main issue with that PR was around its handling of URL dependencies,
so this PR _also_ refactors how we handle those. Previously, we stored
URL dependencies on `PubGrubPackage`, but they were omitted from the
hash and equality implementations of `PubGrubPackage`. This led to some
really careful codepaths wherein we had to ensure that we always visited
URLs before non-URL packages, so that the URL-inclusive versions were
included in any hashmaps, etc. I considered preserving this approach,
but it would require us to rely on lots of internal details of PubGrub
(since we'd now be relying on PubGrub to merge those packages in the
"right" order).

So, instead, we now _always_ set the URL on a given package, whenever
that package was _given_ a URL upfront. I think this is easier to reason
about: if the user provided a URL for `flask`, then we should just
always add the URL for `flask`. If we see some other URL for `flask`, we
error, like before. If we see some unknown URL for `flask`, we error,
like before.

Closes https://github.com/astral-sh/uv/issues/1522.

Closes https://github.com/astral-sh/uv/issues/1821.

Closes https://github.com/astral-sh/uv/issues/1615.

crates/uv-dev/src/resolve_cli.rs

@@ -105,7 +105,7 @@ pub(crate) async fn resolve_cli(args: ResolveCliArgs) -> Result<()> {
         &flat_index,
         &index,
         &build_dispatch,
-    );
+    )?;
     let resolution_graph = resolver.resolve().await.with_context(|| {
         format!(
             "No solution found when resolving: {}",