runtime: Refactor build version information

Previously the build version was recorded in the version package and
then different components would report it in an ad-hoc manner, e.g.,
the REPL has a module that generates a virtual doc with the version
info in it, the server was using templating to do the same, etc.

These changes remove the special code from the REPL and server
implementations to report the version. Instead the runtime writes the
version into /system/version at boot.

Signed-off-by: Torin Sandall <torinsandall@gmail.com>