Remove restriction on defining verbs/types in subpackages

[alecthomas]

I think in general we should use a model of allowing anti-patterns, but warning. In this case the anti-pattern is defining very large numbers of functions and types in a single FTL module.

So instead of preventing types from subpackages:

eg.

unsupported type ftl/foo/bar.Currency from subpackage

Instead, we'd allow this but warn if the number of types/functions defined is over a certain threshold. Of course, due to the single namespace in FTL we would not be able to allow the same type name across multiple subpackages (eg. foo/bar/Currency and foo/waz.Currency), as that will result in an error.