Use a user directory for temporary files

[david butler]

So i think the parts, prime, stage directories should be placed somewhere where you can configure, and I think they should default to `~/.snapcraft/` or something like that.. I can think of multiple advantages:

- You don't clutter the location of the `snapcraft.yaml` file, which can be annoying if that file is under version control
- You can avoid issues where the location of the `snapcraft.yaml` is mounted using a filesystem that doesn't support all the unix capabilities that might be required during building, or avoid any security blocks that might happen because of the uid/gids of the directory
- if the parts are being pulled via git/bzr, you can keep them in the same place, so that later pulls can be incremental...

[Mark Shuttleworth (sabdfl)]

Yes, this seems like a useful option, though it would not be the
default. You would need to think of how to keep them separate for each
snap you are working on. Patch welcome.

  status: triaged

[Cris Dywan (kalikiana)]

I was stumbling over this at first, until I discovered cleanbuild which basically solves this problem (except it doesn't support remote parts at the time of writing).
Might make sense to simply make cleanbuild the default? Also because the "regular" build adds inherent randomness to the produced snap by re-using repositories and even manual tweaks to the system.