#1151: Fix error with InterfaceDirRelative for external paths

[takanuva15]

Description

Please include a summary of the changes and the related issue. Please also include relevant motivation and context.

• Fixes Getting "file not found" error for external package interfaces with v3 but not v2 #1151

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Version of Go used when building/testing:

• 1.26

How Has This Been Tested?

• Unit tests
• Built the binary locally and tested with my MVE repo

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[takanuva15]

@LandonTClipp This should fix #1151. Feel free to edit the PR as needed to your liking

[LandonTClipp]

@takanuva15 could you give me access to your fork? I am adding an e2e test to verify this behaves properly (which it does!).