[repository sources] copy source configuration: source's __init__ modify it while config.sources() return a cached dictionary which shouldn't be modified
This causes pb since f8aa87a773b2 (though it's unclear why it hasn't been a problem before).
Closes #2521848
doc/book usr/share/doc/cubicweb-documentation
doc/html usr/share/doc/cubicweb-documentation
debian/cubicweb-doc usr/share/doc-base/cubicweb-doc