Difficult to use overlay when building snaps

[Michael Terry]

It's my understanding that in order to use the platform content snap, the developer needs to be building their own snap against xenial and the overlay PPA. Correct?

Except using the overlay PPA is kind of a pain:
- On your local machine, that means you're changing your daily work environment.
- You can no longer use "snapcraft cleanbuild" unless you manually edit your chroot's environment.
- When using a Launchpad recipe, you have to remember to manually specify the overlay PPA as a dependent PPA.
- When using build.snapcraft.io, I don't believe you even have the option to specify a PPA (? needs to be confirmed...)

So for a third party developer, these are all obnoxious roadblocks to using our recommended SDK. Do we have a plan for helping 3rd parties that aren't as plugged into how to use PPAs?

==

One solution could be to provide a platform plugin that the platform remote parts might use, because plugins are able to specify a PPA to be added to snapcraft's local apt cache using PLUGIN_STAGE_SOURCES. I haven't done this before, but it is apparently possible.

That way, snapcraft will always use the overlay PPA when building snaps using ubuntu-app-platform.

(Note that ideally snapcraft would additionally allow pinning the overlay PPA above xenial, but that's bug 1654304.)