Fix crash in ctor-eval when a tag is imported

[stevenfontanella]

Fix for fuzzer-detected crash when ctor-eval runs on a module that imports a tag. Prior to #8254, ctor-eval would crash when an imported tag was evaluated, but not when imported. Change the code to allow imported tags even during evaluation.

Note that we can't reason about the identity of imported tags. In the following code, $t1 and $t2 may be the same or different tags:

(import "foo" "bar" (tag $t1))
(import "foo" "bar2" (tag $t2))

In this PR, we assume that $t1 and $t2 are different tags, and that they're the same tag if the import name is the same (this is also not true in general, the hosting environment may provide two different values for the same exact import name). This may cause some correctness issues. As a followup, we can make equality comparison of two imported tags throw FailToEvalException to make evaluation correct.

Part of #8180.

[stevenfontanella]

Ran the fuzzer for ~30 minutes with no errors.