ignore ppx runtime dependencies when implicit transitive deps = false

[c-cube]

Desired Behavior

When (implicit_transitive_deps false) is specified, it would be good if dependencies required by ppx rewriters were excluded from it. For example, ppx_deriving.runtime should be left implicit even in this case, because in all likelihood it's only depended upon by automatically generated code, for which the dependency is already explicit in (pps …).

related: #2733

[rgrinberg]

Could you explain the problem a little more? Why should code generators rely on transitive dependencies. Why can't they specify the full list of deps they intend to use in ppx_runtime_deps?