Need a way to pull build and stage packages from PPAs

[Alistair Buxton]

Example snapcraft.yaml:

https://github.com/ali1234/mysnaps/tree/master/qmlrss

In order to correctly build this snap, several build and stage packages are required which are only available from PPAs. One of the requirements is a patched Qt which is too big to build on the target hardware.

There should be a way to indicate in the yaml that the snap depends on PPAs. This is important for both people who try to randomly build my snap, and also to make cleanbuild work.

[Sergio Schvezov (sergiusens)]

the package-repositories field should support this https://snapcraft.io/docs/package-repositories