don't hang if multiple sources use unavailable method

APT clients always noticed if a method isn't supported and nowadays
generate a message of the form:
	E: The method driver …/foobar could not be found.
	N: Is the package apt-transport-foobar installed?

This only worked if a single source was using such an unavailable method
through as we were registering the failed config the first round and
the second would try to send requests to the not started method, which
wouldn't work and hang instead (+ hiding the error messages as they would
be shown only at the end of the execution).

Closes: 870675

0 files changed, 0 insertions, 0 deletions