[build_manager] introduce BuildArchive

[paulsemel]

This new class removes some of the burden build_manager.py was initially doing. Introducing this class has a few advantages: - Better testing of the unpacking feature. This will not be entangled in the build manager anymore, so this makes it easier to unittest. - Custom build handling. This is true for the current introduction of .runtime_deps, but this could also be true for e.g. listing targets via a file contained in the archive. - Reduce the amount of responsibility of the build_manager.py class. This also reduces what the tests are required to be testing.