fix 'uv pip install' handling of gzip'd response and PEP 691 (#1978)
Thank you for writing `uv`! We're already using it internally on some
container image builds and finding that it's noticeably faster 💯
## Summary
I was attempting to use `uv` alongside [modal](https://modal.com/)'s
internal PyPi mirror and ran into some issues. The first issue was the
following error:
```
error: Failed to download: nltk==3.8.1
Caused by: content-length header is missing from response
```
This error was coming from within
`RegistryClient::wheel_metadata_no_pep658`. By logging requests on the
client (uv) and server (internal mirror) sides I've concluded that it's
occurring because `uv` is sending a header suggesting that it can accept
a gzip'd response, but decompressing the gzip'd response strips the
`content-length` header:
https://github.com/seanmonstar/reqwest/issues/294.
**Logged request, client-side:**
```
0.981664s 0ms INFO uv_client::registry_client JONO, REQ: Request { method: HEAD, url: Url { scheme: "http", cannot_be_a_base: false, username: "", password: None, host: Some(Ipv4(172.21.0.1)), port: Some(5555), path: "/simple/joblib/joblib-1.3.2-py3-none-any.whl", query: None, fragment: None }, headers: {} }
```
No headers set explicitly by `uv`.
**Logged request, server-side:**
```
2024-02-26T03:45:08.598272Z DEBUG pypi_mirror: origin request = Request { method: HEAD, uri: /simple/joblib/joblib-1.3.2-py3-none-any.whl, version: HTTP/1.1, headers: {"accept": "*/*", "user-agent": "uv", "accept-encoding": "gzip, br", "host": "172.21.0.1:5555"}, body: Body(Empty) }
```
Server receives `"accept-encoding": "gzip, br",`.
My change adding the header to the request fixed this issue. But our
internal mirror is just passing through PyPI responses and PyPI
responses do contain PEP 658 data, and so `wheel_metadata_no_pep658`
shouldn't execute.
The issue there is that the PyPi response field has _dashes_ not
_underscores_ (https://peps.python.org/pep-0691/).
<img width="1261" alt="image"
src="https://github.com/astral-sh/uv/assets/12058921/35230f27-441a-457a-827b-870a1a16c16a">
After changing the `alias` the PEP 658 codepath now runs correctly :)
## Test Plan
I tested by installing against both our mirror and against PyPi:
```
RUST_LOG="uv=trace" UV_NO_CACHE=true UV_INDEX_URL="http://172.21.0.1:5555/simple" target/release/uv pip install -v nltk
RUST_LOG="uv=trace" UV_NO_CACHE=true UV_INDEX_URL="http://localhost:5555/simple" target/release/uv pip uninstall -v nltk
```
```
target/release/uv pip install -v nltk
target/release/uv pip uninstall -v nltk
```
