Fix duplicate edges in package resolution mode

[fank]

Summary

Fixes duplicate edges when using package resolution mode. Previously, the tool created one edge per import statement, resulting in multiple duplicate edges between the same package pairs.

Problem

When using --resolution package, if package A had multiple files importing package B, the graph would contain multiple identical edges from A to B (one for each import statement). For large codebases, this resulted in thousands of duplicate edges, making the graph cluttered and misleading.

For example:

• Package internal had 67 files importing package cacheable
• This created 67 identical edges in the output DOT file
• Graph became unreadable with 1815 edges (should have been ~200)

Solution

Added deduplication logic in writeRelationshipsForPackageResolution():

• Tracks unique package-to-package edges using a map
• Each package pair now has exactly one edge
• Prioritizes cycle edges over normal edges when both exist

Results

Testing on a large codebase:

• Before: 1815 edges (with ~1600 duplicates)
• After: 199 edges (all unique)
• Improvement: ~91% reduction in duplicate edges

Testing

Tested on the internal-api codebase with 306 packages:

go run ./cmd/goimportcycle -path ../internal-api/ -dot imports.dot -resolution package

Verified:

• ✅ No duplicate edges in output
• ✅ All package relationships correctly represented
• ✅ Cycle detection still works correctly
• ✅ File resolution mode unaffected

[fank]

-resolution package shows file links:

[samlitowitz]

Resolved by #14