virtual (build) packages are resolved by first match

[Mina Galić]

this can lead to problems when a package such as `tclsh` is provided by `tcl:386` or `tcl:amd64` and the amd64 package is already installed.

for inexplicable reasons, those two packages conflict, so rather than

 the [following line](https://github.com/snapcore/snapcraft/blob/master/snapcraft/internal/repo/_deb.py#L248)

        if apt_cache.is_virtual_package(name):
            name = apt_cache.get_providing_packages(name)[0].name

a better safer approach might be to first filter by `architecture`.

[Mina Galić (minagalic)]

related https://github.com/lxc/lxd-pkg-snap/issues/10