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RFC declarative built-using field generation

The Built-Using field required[1] by recent policy results in some
problems for maintainers:

1. It needs to indicate the exact version of the source package used in
   the build. So this has to be kept up-to-date, or dynamically
   generated. Updating it manually is busywork and won't reflect
   versions on the build daemons; dynamically generating
   it needs some messy scripting, especially in the case of packages
   that embed stuff from dozens of other packages, that may vary by
2. It violates DRY for many packages, since the build dependencies
   already contain similar lists of packages. But not close enough to
   the same to allow Built-Using to be automatically generated.

So, I propose a backwards-compatable hack to the Build-Depends format
that would allow creation of something like dh_buildusing, that can
automatically populate a substvar for the Built-Using field.

We can take advantage of the architecture specification in Build-Depends
being fairly wide-open. So this should not break existing parsers:

Build-Depends: foo [any built-using], bar [i386 amd64 built-using]

Assuming there is never an architecture called "built-using", and that
Build-Depends parsers never hardcode the list of all valid architectures
(which would be prone to break them anyway), this seems an entirely safe
thing to do. The only risk I can think of is that packages.debian.org
would list the "built-using" in its display of build dependencies for
source packages.

With this info declared in the control file, a program can extract the
marked build-depends that match the current architecture, map back to
source packages, and generate the Built-Using field from them.

It might be possible to use some special sigil, like "*" or "^" to mark
built-using, but this seems less likely to work with all Build-Depends
parsers. But if we wanted something shorter than "built-using", we
could use "BU".

Alternatively, a package named built-using could be uploaded. This would

	built-using, foo,
	built-using, bar [i386 amd64]

Where the package following built-using is indicated as needing to go
into Built-Using. This is 100% guaranteed to not cause any breakage
(unless something is really stupid about duplicated package names in
Build-Depends), but is IMHO uglier than the other option.

see shy jo

[1] Or at least encouraged, depending on how policy and the intent of
    policy is interpreted.

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