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/local.rs

@@ -17,6 +17,7 @@ pub async fn build(
     version: &str,
     arch: &str,
     series: &str,
+    pocket: Option<&str>,
     build_root: &str,
     cross: bool,
     ppa: Option<&[&str]>,
@@ -53,7 +54,7 @@ pub async fn build(
     if cross {
         log::debug!("Setting up environment for local cross build...");
         cross::setup_environment(&mut env, arch, ctx.clone())?;
-        cross::ensure_repositories(arch, series, ctx.clone())?;
+        cross::ensure_repositories(arch, series, pocket, ctx.clone())?;
     }
 
     let mut sources = apt::sources::load(Some(ctx.clone()))?;
@@ -116,6 +117,19 @@ pub async fn build(
         }
     }
 
+    // Enable the requested pocket on archive sources, so build-dependencies
+    // are resolved from that pocket
+    if let Some(pocket_name) = pocket {
+        let pocket_suite = format!("{series}-{pocket_name}");
+        log::info!("Enabling pocket '{}' for build dependencies", pocket_suite);
+        for source in &mut sources {
+            if crate::deb::is_archive_source(&source.uri) && !source.suite.contains(&pocket_suite) {
+                source.suite.push(pocket_suite.clone());
+                modified = true;
+            }
+        }
+    }
+
     if modified {
         apt::sources::save_legacy(Some(ctx.clone()), sources, "/etc/apt/sources.list")?;