Support boring on original Module after .toInstance call

[jackkoenig]

Note that simply using the "current ports" in Lookupable is totally fine. It's not possible to see BoringUtils introduced ports via @public so their visibility in the ports seen by Lookupable is irrelevant.

I waffled on if I think this should be legal or not. What eventually convinced me was the following: One of the main reasons .toInstance exists is to write generators that work with both normal Modules and with D/I Instances. What is the purpose of such parameterization in a generator? Obviously, this exists because the user may want to do things that only Modules can do (i.e., that don't work on Instances). One of the most common things in this category is boring, which you can do to Modules but obviously cannot do to Instances. Thus we should make sure it's allowed. I will also note that @azidar in the original PR barred boring after calling .toDefinition, but [I'm assuming] deliberately did not bar it after .toInstance.

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