move to generating a FromCode file

FromCode files have their code evaluated at the same time in-memory files are
evaluated, after the PrereqSource phase but before AfterBuild. Consequently,
all of @kentnl's goals in commit 983bd6bcfc are still fulfilled:

- munge in memory, without the need for slurp and friends
- munging becomes visible to things that run afterwards (in AfterBuild)

Also, it follows the same pattern as other plugins that generate files
dependent on prereq data, such as [MetaJSON] in core, therefore resolving the
alert generated by [VerifyPhases].