Hydrilla

make it easier to reuse/cache foreign package files when building a Hydrilla source package multiple times

always use APT packages for amd64 architecture

raise a more informative error when a file referenced from index.json is missing

properly handle percent-encoded characters in .deb file names

add missing english translations and verify message texts of raised exceptions in tests

support piggybacking on APT packages

use pyproject.toml for pytest configuration and move all tests to tests/ directory