[WIP] Gitian descriptors overhaul

This teaches Gitian to use the new depends system rather than the old build scripts. See #4592 for the reasoning.

Important: This relies on a new caching feature in Gitian. When building (for ex) the linux descriptor, the dependencies will be built and cached. On subsequent builds, if the dependencies have not changed, a cached version will be used rather than rebuilding. For that reason, the descriptors can be a clean one-per-OS. Gitian rev 3a0e9024b or newer is required to take advantage of this.

In order to take advantage of the new cache, sources are no longer listed as inputs. Instead they are handled by the dependency builder. After they've been fetched once, they'll be cached and re-used.

All target libs are cross-built as necessary now, rather than using apt-get to install. That means that a single OS image (in this case Precise x64) can be used for everything.

TODO before merge:
- More docs
- Verify determinism
@Michagogo that's fine, doesn't matter to me.