handle pure virtual packages

With the following buildout.cfg:

    [buildout]
    parts =
	pkgversions

    [pkgversions]
    recipe = c2c.recipe.pkgversions
    libxslt-dev =

    [versions]
    c2c.recipe.pkgversions=0.2dev1

bin/buildout bails out with "Error: The package libxslt-dev is not
installed.".

apt-cache says libxslt-dev is purely virtual:

    # apt-cache show libxslt-dev
    N: Can't select versions from package 'libxslt-dev' as it is purely virtual
    N: No packages found

showpkg shows the following output:

    # apt-cache showpkg libxslt-dev
    Package: libxslt-dev
    Versions:

    Reverse Depends:
      ruby-build,libxslt-dev
    Dependencies:
    Provides:
    Reverse Provides:
    libxslt1-dev 1.1.26-14.1

Running apt-get install gives the following:

    # apt-get install libxslt-dev
    Reading package lists... Done
    Building dependency tree
    Reading state information... Done
    Note, selecting 'libxslt1-dev' instead of 'libxslt-dev'
    libxslt1-dev is already the newest version.
    0 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.

I know how to workaround that problem, however it would be nice if
c2c.recipe.pkgversions would be able to handle that for me.