Fixes for puku init step & codelab

[peterebden]

Main thing is getting plz init plugin go to just write the filegroup and set modfile, which should avoid the main Puku papercut where it just silently does nothing (that behaviour also needs a fix in Puku really, it exits with 0 in too many unsuccessful cases).

Also some minor changes to the codelab as I ran through it:

• Remove the version pin. It gets out of date and the codelab should be happy to just use the latest.
• Don't label the puku alias as optional, because the rest of the codelab references plz puku so it won't work without. (We might also want plz init plugin go to just do this?)
• Remove the go install std directive. We don't need it any more (we build it via plz on-demand)
• Remove the initial "sync the changes" bit; it doesn't sync at that point for me. I think you need to run plz puku fmt first.

Fixes #3534