Draft: Collect arbitrary package-defined artifacts such as extra logs

See the thread on -devel starting at: https://lists.debian.org/debian-devel/2022/02/msg00216.html

Requires !120 (merged) (otherwise the new documentation would conflict). Untested.

• Build: Factor out a function to create filenames in the log directory

  This will let us save other build artifacts such as detailed test results alongside the log.

• Build: Collect additional artifacts from each build attempt

  This is primarily intended as a way to collect detailed test results and other information about test status, replacing the mechanisms currently used by binutils/gcc (packaging detailed test logs in /usr/share/doc) and gtk4 (uuencoding image comparisons and putting them in the build log). It could also be used to collect non-test logs such as Autotools config.log.

  Artifacts are collected for all builds, whether successful or failed, so that they can be used to diagnose test failures: this means they are treated as being more like the log file than like the build products (in particular they are not listed in buildinfo or changes files).

  This uses the same patterns as copyright-format 1.0 (whitespace-separated patterns, '*' and '?' match anything, '.' and '/' are not treated specially by wildcards, '[' is treated as literal).

  As prototyped here, the artifacts end up in a tarball with the same name as the log file, except that '.build' is replaced with '-artifacts.tar.gz', for example libportal_0.5-2_amd64-2022-02-16T13:09:39Z-artifacts.tar.gz.