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>

Cargo.toml

@@ -27,6 +27,7 @@ xz2 = "0.1"
 serde_json = "1.0.145"
 directories = "6.0.0"
 ssh2 = "0.9.5"
+tempfile = "3.10.1"
 
 [dev-dependencies]
-tempfile = "3.10.1"
+test-log = "0.2.19"