[BUG] - Validation of omlxmi files does not report missing imports

[dwagmuse]

Description

Serializer for omlxmi files will fail to write file if imports are not complete. However, validation does not report this as an error prior to actually trying to write the file

Steps to Reproduce

This occurs most often when creating new omlxmi files or converting from earlier (0.9x) versions where imports were not required.
It may not be a problem in new files created originally in OML tree editor (haven't tried that).

Open omlxmi file that has references to other namespace but no import for it.
Validate file (should succeed)
Make some minor value edit to cause save to rewrite the file
Save
This should produce an exception reporting the missing export

Expected Behavior

It would be most helpful if the validation process were to report these missing imports during validation (and maybe even fix them up automatically, or at least suggest the fix).

Additional Context

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Relevant screenshots

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