deb: cross-compilation, ephemeral contexts, local builds

Multiple changes:
- New contexts (schroot, unshare)
- Cross-building quirks, with ephemeral contexts and repositories management
- Contexts with parents, global context manager, better lifetime handling
- Local building of binary packages
- Pull: pulling dsc files by default
- Many small bugfixes and changes

Co-authored-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Co-committed-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>

src/package_info.rs

@@ -105,7 +105,7 @@ pub async fn get_dist_series(dist: &str) -> Result<Vec<String>, Box<dyn Error>>
     }
 }
 
-async fn get_dist_from_series(series: &str) -> Result<String, Box<dyn Error>> {
+pub async fn get_dist_from_series(series: &str) -> Result<String, Box<dyn Error>> {
     let debian_series = get_dist_series("debian").await?;
     if debian_series.contains(&series.to_string()) {
         return Ok("debian".to_string());