depends: Gitian 0.10 fixes

Among other things, fixes #5428. Since this will cause all dependencies to rebuild, I cleaned up a few other things while I was in there:
- `make download` now downloads sources for all OSs.
- Updated docs to add `make download` before gitian build
- fixed the comparisontool install process, which was moving the downloaded jar file
- moved download stamps into the SOURCES_PATH dir, so they're always in sync
- make build-id's deterministic by normalizing sha256 output for patches
- other misc fixes and doc cleanups

Verified that when fetching the sources before building, gitian does not attempt to download anything.