merge sources.list lines based on Release filename

merge sources.list lines based on Release filename

Merging by URI means that having sources lines with different URI
methods results in 'strange' warning and error messages, which aren't
very friendly from a user point of view as not encoding the method in
the filename is effectivly an implementation detail.

Merging by filename removes these messages and makes everything "work"
even if it isn't working the way it is configured as the indexes aren't
acquired over the method given, but over the first method for this
release file (which argueably is an implementation detail stemming from
the filename encoding, too).

So either direction isn't perfectly "right", but personally I prefer
"magic" over strange error messages (and doing a full-circle detection
of this with its own messages which would need to be translated feels
like way too much effort for dubious gain).

Closes: 826944