Hydrilla

add tests for features from version 2 of source package JSON schema

incorporate version 2 of Hydrilla JSON schemas

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

make pytest try to load the tested modules from sources if they are present

add babel to extras_require