Avoid duplicating authorization header with netrc (#2325)
## Summary
The netrc middleware we added in
https://github.com/astral-sh/uv/pull/2241 has a slight problem. If you
include credentials in your index URL, _and_ in the netrc file, the
crate blindly adds the netrc credentials as a header. And given the
`ReqwestBuilder` API, this means you end up with _two_ `Authorization`
headers, which always leads to an invalid request, though the exact
failure can take different forms.
This PR removes the middleware crate in favor of our own middleware.
Instead of using the `RequestInitialiser` API, we have to use the
`Middleware` API, so that we can remove the header on the request
itself.
Closes https://github.com/astral-sh/uv/issues/2323.
## Test Plan
- Verified that running against a private index with credentials in the
URL (but no netrc file) worked without error.
- Verified that running against a private index with credentials in the
netrc file (but not the URL) worked without error.
- Verified that running against a private index with a mix of
credentials in both _also_ worked without error.
