Bug#878961: debootstrap fails to resolve dependencies on virtual packages

[Daniel Drake <drake@endlessm.com>]

Package: debootstrap
Version: 1.0.81ubuntu3.2
Severity: normal

debootstrap is unable to operate if the packages being included have a
virtual dependency which is not otherwise resolved.

Easy reproducer:
  # debootstrap --include=libnet-ssleay-perl stretch stretch
  [...]
  W: Failure while configuring base packages.  This will be re-attempted up to five times.
  W: See debootstrap.log for details (possibly the package libnet-ssleay-perl is at fault)

The log shows error:
  dpkg: dependency problems prevent configuration of libnet-ssleay-perl:
   libnet-ssleay-perl depends on perl-openssl-abi-1.1; however:
    Package perl-openssl-abi-1.1 is not installed.

libnet-ssleay-perl depends on perl-openssl-abi-1.1, which is a virtual package
provided by perl-openssl-defaults.

debootstrap's pkgdetails_perl dependency resolution mechanism does not
consider virtual packages, so the dependency on perl-openssl-abi-1.1 is
basically ignored.

The expected outcome is for it to be resolved to perl-openssl-defaults,
which would then be installed along with the other packages and result in a
usable bootstrap.

I've reproduced this on v1.0.81ubuntu3.2 but from code inspection I can see
the same problem exists in current git - nothing examines Provides here.

Also reported at
https://bugs.launchpad.net/ubuntu/+source/debootstrap/+bug/86536

-- System Information:
Debian Release: stretch/sid
  APT prefers zesty-updates
  APT policy: (500, 'zesty-updates'), (500, 'zesty-security'), (500, 'zesty'), (100, 'zesty-backports')
Architecture: amd64 (x86_64)
Foreign Architectures: i386

Kernel: Linux 4.10.0-26-generic (SMP w/4 CPU cores)
Locale: LANG=en_GB.UTF-8, LC_CTYPE=en_GB.UTF-8 (charmap=UTF-8)
Shell: /bin/sh linked to /bin/dash
Init: systemd (via /run/systemd/system)

Versions of packages debootstrap depends on:
ii  wget  1.18-2ubuntu1

Versions of packages debootstrap recommends:
ii  gnupg           2.1.15-1ubuntu7
ii  ubuntu-keyring  2016.10.27

debootstrap suggests no packages.

-- no debconf information