fix: fix module mappings logic for deps that have module_name but no module_root

[gregmagolan]

Fix logic bug in module_mappings.bzl.

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• Tests for the changes have been added (for bug fixes / features)
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What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
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What is the current behavior?

Issue Number: N/A

What is the new behavior?

Does this PR introduce a breaking change?

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• No

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