This reduces the need to have a giant metapackage with all the shared
libraries' detached debug symbols; now we only need to do that for
libraries that are still using legacy debug symbols (foo-dbg).
This also means we tolerate transient uninstallability for debug
symbols: we'll simply produce a runtime without those debug symbols.
This is particularly important when taking binaries from
debian-security, which does not have a corresponding debug symbols
archive (although when security updates get mirrored into
proposed-updates, their detached debug symbols do appear there).
Finally, it also means we don't run into problems with version skew
between debug symbols and the actual libraries, particularly when
adding buster-proposed-updates-debug (which, in general, will be
strictly newer than buster + buster-security) in the hope of getting
detached debug symbols for buster-security.
Signed-off-by: Simon McVittie <firstname.lastname@example.org>