Bug#991962: src: prefix should work also with experimental source only

[Patrice Duroux]

Ok I got it. My apt config was missing the corresponding 'deb-src' for the
experimental release and so apt-cache does not found it.
I suspect that taking into account the 'Source:' field could not be a nicer
(less restrictive) solution.
Feel free to close this issue then.
Thanks!