[sources management] fix crash mapping tab because chain() is not indexable. Closes #2124077
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/devtools/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/skeleton/ usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/test usr/lib/PY_VERSION/site-packages/cubicweb/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/entities/test usr/lib/PY_VERSION/site-packages/cubicweb/entities/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/ext/test usr/lib/PY_VERSION/site-packages/cubicweb/ext/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/server/test usr/lib/PY_VERSION/site-packages/cubicweb/server/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/hooks/test usr/lib/PY_VERSION/site-packages/cubicweb/sobjects/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/web/test usr/lib/PY_VERSION/site-packages/cubicweb/web/
debian/tmp/usr/lib/PY_VERSION/site-packages/cubicweb/etwist/test usr/lib/PY_VERSION/site-packages/cubicweb/etwist/