Refactor/move plugin package up

[lchoquel]

• Moved “plugin” to the same level as “cogt” within pipelex for better visibility
• Major cleanup in the unit tests: hierarchy largely flattened
• Added pytest-xdist to run unit tests in parallel on multiple CPUs. Not yet integrated into the Makefile, so run it manually with pytest -n auto or pytest -n auto -m "llm"
• Added make test-quiet (shorthand tq) to Makefile to run tests without capturing outputs (i.e. without pytest -s option)
• Stopped using Fixtures for pipe_router and content_generator: we're now always getting the singleton from pipelex.hub

[thomashebrard]

good

[antoinepipelex]

lgtm