distro_info: add UNRELEASED series handling, use it in deb and chlog

src/deb/mod.rs

@@ -83,8 +83,20 @@ async fn build_binary_package_impl(
     let changelog_path = cwd.join("debian/changelog");
     let (package, version, package_series) =
         crate::changelog::parse_changelog_header(&changelog_path)?;
+    // UNRELEASED is not a real archive series: without an explicit --series,
+    // build against the development series of the host vendor's distribution
+    // instead. An explicit --series always wins.
+    let resolved_series;
     let series = if let Some(s) = series {
         s
+    } else if crate::distro_info::is_unreleased(&package_series) {
+        let dist = crate::build::env::current_vendor();
+        resolved_series = crate::distro_info::effective_series(&package_series, &dist).await?;
+        log::info!(
+            "Changelog is UNRELEASED, building against series {}",
+            resolved_series
+        );
+        resolved_series.as_str()
     } else {
         &package_series
     };