With compat 14, it will be possible to automatically add a `Provides`
field and therefore it becomes much more interesting to automate
features like this.

What we want here is that exactly one package has a `Provides:
dh-sequence-foo`. For the 80% case, the package containing `foo.pm` will
be the one with the `Provides`. However, there is at least the exception
where `libdebhelper-perl` contains all the sequences provided by
`debhelper` and you really need the `debhelper` package for them to work.

Perhaps we can get away with the logic:

  * Check if any binary from the same source provides dh-sequence-foo.
    - If no, add dh-sequence-foo to the Provides of the package
      containing foo.pm.
    - If yes, skip foo.pm

This would work for `debhelper` and any similar case where the relevant
packages are built from the same source. I think this would be
acceptable in practice.

Best regards,
Niels