Add interactive provider selection

If there are multiple providers in one db, pacman used to just stop at
the first one (both during dependency resolution or for pacman -S
'provision' which uses the same code).

This adds a new conversation callback so that the user can choose which
provider to install. By default (user press enter or --noconfirm), the
first provider is still chosen, so for example the behavior of sync402
and 403 is preserved. But at least the user now has the possibility to
make the right choice in a manual run.

If one of the provider is already installed, it is picked for
reinstall/upgrade, so that provision 002/003 pactest now pass.

$ pacman -S community/smtp-server
:: There are 3 providers available for smtp-server:
   1) courier-mta  2) esmtp  3) exim

Which one do you want to install?
Enter a number (default=1):

Signed-off-by: Xavier Chantry <chantry.xavier@gmail.com>

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