Investigate use of default target namespace in import frontend

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Follows up on !190598 (merged).

Can we remove the use of elsewhere in the code, now that the store no longer provides a default:

• defaultTargetNamespace
• default_target_namespace

See !190598 (comment 2525189288) !190598 (comment 2525189296)

Other cleanup:

• !190598 (comment 2525189271)
• !190598 (comment 2525189279)