Code blocks don't work well in godoc for variables because they render the same
as the "var Name = Value" blocks themselves, resulting in a confusing look.
Also make the instructions in PACKAGING.md even briefer, now that the full
instruction is available as a continuous prose in the godoc for pkg/buildinfo.
This allows the build commands in Makefile and tools/buildall.sh to be
simplified.
Official reproducible builds are now handled as a build variant, and the command
in Makefile no longer tries to produce reproducible builds.
Instructions in PACKAGING.md have been completely rewritten.
Introduce `builtin:buildinfo` and `builtin:version` vars. This also
changes the `-buildinfo -json` implementation to use the standard
encoding/json package rather than handcrafting the JSON string.
This also fixes three incorrect spellings of "overridden". Normally I would
do those in a separate change but since there are only three instances,
one of which one was legitimately part of this change, I decided to bundle
the other two.
The mechanism is now documented in PACKAGING.md.
Also refactor tools/buildall.sh to make it easy to make reproducible builds, and
fix tools/cirrus-deploy.sh.
