Allow non-nested archives for hexdump and others (#1564)

## Summary#1562 

It turns out that `hexdump` uses an invalid source distribution format
whereby the contents aren't nested in a top-level directory -- instead,
they're all just flattened at the top-level. In looking at pip's source
(51de88ca64/src/pip/_internal/utils/unpacking.py (L62)),
it only strips the top-level directory if all entries have the same
directory prefix (i.e., if it's the only thing in the directory). This
PR accommodates these "invalid" distributions.

I can't find any history on this method in `pip`. It looks like it dates
back over 15 years ago, to before `pip` was even called `pip`.

Closes https://github.com/astral-sh/uv/issues/1376.

crates/uv-distribution/src/source/mod.rs

@@ -787,7 +787,11 @@ impl<'a, T: BuildContext> SourceDistCachedBuilder<'a, T> {
         drop(span);
 
         // Extract the top-level directory.
-        let extracted = uv_extract::strip_component(temp_dir.path())?;
+        let extracted = match uv_extract::strip_component(temp_dir.path()) {
+            Ok(top_level) => top_level,
+            Err(uv_extract::Error::NonSingularArchive(_)) => temp_dir.into_path(),
+            Err(err) => return Err(err.into()),
+        };
 
         // Persist it to the cache.
         fs_err::tokio::create_dir_all(cache_path.parent().expect("Cache entry to have parent"))