raspbmirror: add --nosources option

[mutability]

As the name implies this disables mirroring of source packages and the Sources file

[plugwash]

i'm reluctant here, mirroring without sources puts you at high risk of violating licenses like the GPL. If you really want to do it there are other tools you can use.

[mutability]

This was mostly to re-bootstrap a mirror in a reasonable time (where the main reason for existing was the binary packages), not a long-term thing. Then let it backfill the sources later.

What other tool would you prefer us to use for mirroring? I thought raspbmirror was the preferred option.

[plugwash]

raspbmirror is the preferred tool for making complete mirrors of the raspbian archive, including some bits that other tools will miss (for example extra source packages, needed to satisfy "built-using", and files outside the apt archive). I wrote it because rsync was causing severe server load issues.