Support for "base: core"

[Sergio Schvezov]

New features of snapcraft are only triggered by the use of the base keyword.
Developers targeting 16.04 as a base should be using core16, but considering that core16 is not ready yet we shall add support for use of core as a base and remove it from the resulting snap.yaml to not affect the internal snapd logic as core is not of type base (even though it is conceptually a base).

For launchpad we will need to ensure that when "base: core" is found in snapcraft.yaml, that the snapcraft snap is used instead of the deb (the logic in general terms should be, if the base keyword is found, use the snapcraft snap, else the deb).

[Sergio Schvezov (sergiusens)]

https://github.com/snapcore/snapcraft/pull/2499

[Colin Watson (cjwatson)]

I've reconfigured Launchpad to match this, for now just by creating a new pseudo-base to act as the default for snaps that don't declare a base instead of using "core" for that:

>>> legacy_default = lp.snap_bases.new(name='legacy-default', display_name='Legacy default', distro_series='/ubuntu/xenial', build_channels={'snapcraft': 'apt'})
>>> lp.snap_bases.setDefault(snap_base=legacy_default)
>>> core = lp.snap_bases.getByName(name='core')
>>> core.build_channels = {'snapcraft': 'stable/launchpad-buildd'}
>>> core.lp_save()