Build backend: Add functions to collect file list (#9602)

Using the directory writer trait, we can collect the files instead of
writing them to a real sink. This builds up to a `uv build --list`
similar to `cargo package --list`. It is not connected to the cli yet.

crates/uv-build-backend/src/source_dist.rs

@@ -1,6 +1,6 @@
 use crate::metadata::{BuildBackendSettings, DEFAULT_EXCLUDES};
 use crate::wheel::build_exclude_matcher;
-use crate::{DirectoryWriter, Error, PyProjectToml};
+use crate::{DirectoryWriter, Error, FileList, ListWriter, PyProjectToml};
 use flate2::write::GzEncoder;
 use flate2::Compression;
 use fs_err::File;
@@ -35,6 +35,26 @@ pub fn build_source_dist(
     Ok(filename)
 }
 
+/// List the files that would be included in a source distribution and their origin.
+pub fn list_source_dist(
+    source_tree: &Path,
+    uv_version: &str,
+) -> Result<(SourceDistFilename, FileList), Error> {
+    let contents = fs_err::read_to_string(source_tree.join("pyproject.toml"))?;
+    let pyproject_toml = PyProjectToml::parse(&contents)?;
+    let filename = SourceDistFilename {
+        name: pyproject_toml.name().clone(),
+        version: pyproject_toml.version().clone(),
+        extension: SourceDistExtension::TarGz,
+    };
+    let mut files = FileList::new();
+    let writer = ListWriter::new(&mut files);
+    write_source_dist(source_tree, writer, uv_version)?;
+    // Ensure a deterministic order even when file walking changes
+    files.sort_unstable();
+    Ok((filename, files))
+}
+
 /// Build includes and excludes for source tree walking for source dists.
 fn source_dist_matcher(
     pyproject_toml: &PyProjectToml,