Add support for URL dependencies (#251)

## Summary

This PR adds support for resolving and installing dependencies via
direct URLs, like:

```
werkzeug @ 960bb4017c/Werkzeug-2.0.0-py3-none-any.whl
```

These are fairly common (e.g., with `torch`), but you most often see
them as Git dependencies.

Broadly, structs like `RemoteDistribution` and friends are now enums
that can represent either registry-based dependencies or URL-based
dependencies:

```rust
/// A built distribution (wheel) that exists as a remote file (e.g., on `PyPI`).
#[derive(Debug, Clone)]
#[allow(clippy::large_enum_variant)]
pub enum RemoteDistribution {
    /// The distribution exists in a registry, like `PyPI`.
    Registry(PackageName, Version, File),
    /// The distribution exists at an arbitrary URL.
    Url(PackageName, Url),
}
```

In the resolver, we now allow packages to take on an extra, optional
`Url` field:

```rust
#[derive(Debug, Clone, Eq, Derivative)]
#[derivative(PartialEq, Hash)]
pub enum PubGrubPackage {
    Root,
    Package(
        PackageName,
        Option<DistInfoName>,
        #[derivative(PartialEq = "ignore")]
        #[derivative(PartialOrd = "ignore")]
        #[derivative(Hash = "ignore")]
        Option<Url>,
    ),
}
```

However, for the purpose of version satisfaction, we ignore the URL.
This allows for the URL dependency to satisfy the transitive request in
cases like:

```
flask==3.0.0
werkzeug @ 254c3e9b5f/werkzeug-3.0.1-py3-none-any.whl
```

There are a couple limitations in the current approach:

- The caching for remote URLs is done separately in the resolver vs. the
installer. I decided not to sweat this too much... We need to figure out
caching holistically.
- We don't support any sort of time-based cache for remote URLs -- they
just exist forever. This will be a problem for URL dependencies, where
we need some way to evict and refresh them. But I've deferred it for
now.
- I think I need to redo how this is modeled in the resolver, because
right now, we don't detect a variety of invalid cases, e.g., providing
two different URLs for a dependency, asking for a URL dependency and a
_different version_ of the same dependency in the list of first-party
dependencies, etc.
- (We don't yet support VCS dependencies.)

crates/puffin-resolver/src/error.rs

@@ -1,5 +1,6 @@
 use pubgrub::range::Range;
 use thiserror::Error;
+use url::Url;
 
 use pep508_rs::Requirement;
 
@@ -25,13 +26,21 @@ pub enum ResolveError {
     #[error(transparent)]
     PubGrub(#[from] pubgrub::error::PubGrubError<PubGrubPackage, Range<PubGrubVersion>>),
 
-    #[error("Failed to build source distribution {filename}")]
-    SourceDistribution {
+    #[error("Failed to build distribution: {filename}")]
+    RegistryDistribution {
         filename: String,
         // TODO(konstin): Gives this a proper error type
         #[source]
         err: anyhow::Error,
     },
+
+    #[error("Failed to build distribution: {url}")]
+    UrlDistribution {
+        url: Url,
+        // TODO(konstin): Gives this a proper error type
+        #[source]
+        err: anyhow::Error,
+    },
 }
 
 impl<T> From<futures::channel::mpsc::TrySendError<T>> for ResolveError {