The resulting `boulder` binary can be invoked by different names to
trigger the behavior of the relevant subcommand. For instance, symlinking
and invoking as `boulder-ca` acts as the CA. Symlinking and invoking as
`boulder-va` acts as the VA.
This reduces the .deb file size from about 200MB to about 20MB.
This works by creating a registry that maps subcommand names to `main`
functions. Each subcommand registers itself in an `init()` function. The
monolithic `boulder` binary then checks what name it was invoked with
(`os.Args[0]`), looks it up in the registry, and invokes the appropriate
`main`. To avoid conflicts, all of the old `package main` are replaced
with `package notmain`.
To get the list of registered subcommands, run `boulder --list`. This
is used when symlinking all the variants into place, to ensure the set
of symlinked names matches the entries in the registry.
Fixes#5692
Jacob Hoffman-Andrews