prevent a dependency from swh.loader.core to swh.loader.package

Having loader.core depending on loader.package makes no sense, and should be prohibited.

This pulls a number a strings to be done properly, and comes with:

• some rework of the test scaffolding (move conftest contents, remove non-pytest functions from conftests, remove the top-level conftest)
• remove swh/loader/init.py and swh/loader/package/init.py files (this later because it makes sense to think as swh.loader.package as a namespace package rather than a module)
• merge swh.loader.package's 'utils.py' with swh.loade.core one; there is no real need for a dedicated utils in package when loader/core as a whole is a bunch of common utility declarations for loaders.