update obsolete dependency on btrfs-tools

[Scott Moser]

Per Adam in #ubuntu-devel:

18:45 <infinity> smoser: curtin's btrfs-tools dep is muy obsolete. Can you update it to either btrfs-progs or "brtfs-progs | btrfs-tools" if you're aiming for backportability?

In addition to debian/control we need to fix any in-target fixes. package installation too, which will be a bigger pita.

$ grep -r --exclude "*.pyc" btrfs-tools *
curtin/block/__init__.py: 'btrfs': ['btrfs-tools'],
curtin/deps/__init__.py: ('mkfs.btrfs', 'btrfs-tools'),
debian/control: btrfs-tools | btrfs-progs,
tests/unittests/test_curthooks.py: ('lvm2', 'xfsprogs', 'btrfs-tools')),
tests/unittests/test_curthooks.py: ('lvm2', 'mdadm', 'e2fsprogs', 'btrfs-tools')),
tests/unittests/test_curthooks.py: ('e2fsprogs', 'btrfs-tools', 'vlan', 'ifenslave')),

[Adam Conrad (adconrad)]

Upgrading importance to high (arguably, it's critical?) because btrfs-tools is NBS and scheduled to be removed from the archive. The only reason it hasn't been yet is to avoid breaking curtin.

[Dimitri John Ledkov (xnox)]

Actually, we'd rather not.

btrfs-progs (4.16.1-2ubuntu1) cosmic; urgency=medium

  * Add Provides: btrfs-tools, such that curtin can continue to install
    btrfs-tools.

 -- Dimitri John Ledkov <email address hidden> Wed, 23 May 2018 12:42:19 +0100

I've added the provides back in. The dep should be resolvable.

The problem is, curtin needs to be installable and be able to install, btrfs-* on all currently supported releases, and the easiest way to achieve that is to use the "compatible" btrfs-tools name across all releases.

Shouldn't it be safe to remove btrfs-tools from cosmic, given above?!