Switch from vendored PubGrub to a fork (#438)

A fork will let us stay up to date with the upstream while replaying our
work on top of it.

I expect a similar workflow to the RustPython-Parser fork we maintained,
except that I wrote an automation to create tags for each commit on the
fork (https://github.com/zanieb/pubgrub/pull/2) so we do not need to
manually tag and document each commit.

To update with the upstream:

- Rebase our fork's `main` branch on top of the latest changes in
upstream's `dev` branch
- Force push, overwriting our `main` branch history
- Change the commit hash here to the last commit on `main` in our fork

Since we automatically tag each commit on the fork, we should never lose
the commits that are dropped from `main` during rebase.

crates/puffin-resolver/Cargo.toml

@@ -16,7 +16,6 @@ pep440_rs = { path = "../pep440-rs" }
 pep508_rs = { path = "../pep508-rs" }
 platform-host = { path = "../platform-host" }
 platform-tags = { path = "../platform-tags" }
-pubgrub = { path = "../../vendor/pubgrub" }
 puffin-cache = { path = "../puffin-cache" }
 puffin-client = { path = "../puffin-client" }
 puffin-distribution = { path = "../puffin-distribution" }
@@ -37,6 +36,7 @@ fxhash = { workspace = true }
 itertools = { workspace = true }
 once_cell = { workspace = true }
 petgraph = { workspace = true }
+pubgrub = { workspace = true }
 tempfile = { workspace = true }
 thiserror = { workspace = true }
 tokio = { workspace = true }