transpile: fix zero initializer imports

[thedataking]

Fix zero-initializer import replay when a cached initializer is reused from a reorganized submodule.

The initializer type import now participates in the deferred import set that is cached with the initializer. This prevents typedef names such as STAT_T from being emitted in a header module without the matching use.

Surfaced when working on PR #1861.

[fw-immunant]

From 1:1 discussion: I don't understand why this isn't a semantics-preserving change, given that beforehand we call import_type upfront (even on calls that will be cache hits), and afterward we defer imports and then do so (so that calling import_type does not directly modify the imports when called but rather adds these imports to the pending cache entry). It seems like both before and after this change, the imports added by import_type will be added on every call to zero_initializer and the only thing changing is whether they are read from the cache or not.

I'd like to see an explanation of what isn't true in the above understanding, or an example trace of how control/data flow works out for a particular case that makes it clear why this is a fix.

Does the attached test really fail without the accompanying change to zero_initializer?