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/build.rs

@@ -19,7 +19,6 @@ pub fn build_source_package(cwd: Option<&Path>) -> Result<(), Box<dyn Error>> {
 
 #[cfg(test)]
 mod tests {
-    use super::*;
 
     // We are not testing the build part, as for now this is just a wrapper
     // around dpkg-buildpackage.