Fix OpaqueSlot handling of contextual names

[jackkoenig]

We need to ensure that contextual names stay contextual (ie. sensitive
to the module context which is important for naming ports).

Contributor Checklist

• Did you add Scaladoc to every public function/method?
• Did you add at least one test demonstrating the PR?
• Did you delete any extraneous printlns/debugging code?
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This is a bug fix in a not-yet-released feature

Type of Improvement

• bug fix

API Impact

No impact, just a minor bug fix

Backend Code Generation Impact

No impact

Desired Merge Strategy

• Squash

Release Notes

Reviewer Checklist (only modified by reviewer)

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• Did you mark the proper milestone (Bug fix: 3.4.x, [small] API extension: 3.5.x, API modification or big change: 3.6.0)?
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