Improve error message for invalid sdist archives (#1389)
This PR improves the error message for the problem described in
https://github.com/astral-sh/uv/issues/1376. The original output
duplicates the actual error message and includes lots of noise
(`DirEntry { inner: DirEntry(...) }`).
```
$ uv pip install hexdump==3.3
error: Failed to download and build: hexdump==3.3
Caused by: Failed to extract source distribution: The top level of the archive must only contain a list directory, but it contains: [DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/__main__.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/hexdump.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/data") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/PKG-INFO") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/setup.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/README.txt") }]
Caused by: The top level of the archive must only contain a list directory, but it contains: [DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/__main__.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/hexdump.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/data") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/PKG-INFO") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/setup.py") }, DirEntry { inner: DirEntry("/home/robin/.cache/uv/.tmpgSvTCk/README.txt") }]
```
This PR removes the duplication and `DirEntry` internals so that the
error message is easier to grasp:
```
$ uv pip install hexdump==3.3
error: Failed to download and build: hexdump==3.3
Caused by: Failed to extract source distribution
Caused by: The top level of the archive must only contain a list directory, but it contains: ["__main__.py", "hexdump.py", "data", "PKG-INFO", "setup.py", "README.txt"]
```
