Bug#780151: debian-installer: Buggy Built-Using generation on an arch-dep manner

[Cyril Brulebois <kibi@debian.org>]

Source: debian-installer
Severity: important

Built-Using was missing some items, see #696418 & #700026, so a patch
was pushed to try and deal with it:

    commit b4411bfb56601cf6a7366a1bec065b56b91f1a42
    Author: Cyril Brulebois <kibi@debian.org>
    Date:   Mon Mar 3 00:00:46 2014 +0100
    
        Extend write-built-using to also generate the ${extra:Built-Using} substvar.
        
        This makes it possible to track packages mentioned in the EXTRA_PACKAGES
        variable (currently set to "bf-utf-source syslinux"). Thanks to Ansgar
        Burchardt for the reports (Closes: #696418, #700026).
        
        Note: Unknown packages are ignored, so architecture-specific packages
        shouldn't be a problem.
        
        Acked-by: Ian Campbell <ijc@debian.org>
        Signed-off-by: Cyril Brulebois <kibi@debian.org>

I lost track of Karsten's objection here:
  https://lists.debian.org/debian-boot/2015/02/msg00012.html

so here's a bug report to try and get that fixed instead of just
reverting the said commit. (Funny how things that shouldn't be an issue
turn out to be one…)

It's indeed sad that I only tested the case of an entirely inexistent
package and didn't think about the funny not-available-on-all-archs
packages…

Maybe we could iterate over the packages mentioned there, calling
dpkg-checkbuilddeps with its -d flag to see if the relevant packages are
relevant? Another idea might be to use Dpkg::Deps to figure out whether
some package is relevant for the current architecture? Any other ideas?

I'd happily take patches here, as I'm busy with other bugs.

Mraw,
KiBi.