Flatten requirements eagerly in get_dependencies (#4430)

Downstack PR: #4515 Upstack PR: #4481

Consider these two cases:

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

B:
```toml
dependencies = [
  "iniconfig == 1.1.1 ; python_version < '3.12'",
  "iniconfig @ git+https://github.com/pytest-dev/iniconfig@93f5930e668c0d1ddf4597e38dd0dea4e2665e7a ; python_version >= '3.12'",
]
```

In the first case, `werkzeug==2.0.0` should be overridden by the url. In
the second case `iniconfig == 1.1.1` is in a different fork and must
remain a registry distribution.

That means the conversion from `Requirement` to `PubGrubPackage` is
dependent on the other requirements of the package. We can either look
into the other packages immediately, or we can move the forking before
the conversion to `PubGrubDependencies` instead of after. Either version
requires a flat list of `Requirement`s to use. This refactoring gives us
this list.

I'll add support for both of the above cases in the forking urls branch
before merging this PR. I also have to move constraints over to this.

crates/uv-configuration/src/constraints.rs

@@ -16,7 +16,11 @@ impl Constraints {
             constraints
                 .entry(requirement.name.clone())
                 .or_default()
-                .push(requirement);
+                .push(Requirement {
+                    // We add and apply constraints independent of their extras.
+                    extras: vec![],
+                    ..requirement
+                });
         }
         Self(constraints)
     }