move to generating a FromCode file

FromCode files have their code evaluated at the same time in-memory files are
evaluated, after the PrereqSource and InstallTool phases but before AfterBuild.
Consequently, the file's existence becomes visible to other plugins that run in
the file munging phase.  It also 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].