pull,deb: add top-level --pocket option

- pull: select the pocket to download the source package from
- deb: build against dependencies from a specific pocket:
  - local mode: enable '{series}-{pocket}' suite on archive sources
  - cross mode: include the pocket suite in required repositories

src/deb/mod.rs

@@ -19,6 +19,7 @@ pub enum BuildMode {
 pub async fn build_binary_package(
     arch: Option<&str>,
     series: Option<&str>,
+    pocket: Option<&str>,
     cwd: Option<&Path>,
     cross: bool,
     mode: Option<BuildMode>,
@@ -99,6 +100,7 @@ pub async fn build_binary_package(
                 &version,
                 arch,
                 series,
+                pocket,
                 &build_root,
                 cross,
                 ppa,
@@ -107,7 +109,7 @@ pub async fn build_binary_package(
             )
             .await?
         }
-    };
+    }
 
     // Retrieve produced .deb files
     let remote_files = build_ctx.list_files(Path::new(&build_root))?;
@@ -256,6 +258,15 @@ fn find_dsc_file(
     Ok(dsc_path)
 }
 
+/// Check whether an apt source URI points to a distribution archive
+/// (as opposed to a PPA or another third-party repository)
+pub(crate) fn is_archive_source(uri: &str) -> bool {
+    uri.contains("archive.ubuntu.com")
+        || uri.contains("security.ubuntu.com")
+        || uri.contains("ports.ubuntu.com")
+        || uri.contains("deb.debian.org")
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -303,6 +314,7 @@ mod tests {
         crate::deb::build_binary_package(
             arch,
             Some(series),
+            None,
             Some(&cwd),
             cross,
             None,