tools: Clean returned paths in ListPkgsUnderDir
tools: Clean returned paths in ListPkgsUnderDir

We would packages with a trailing `/`. This would trip up other tools slightly,
leading them to doing more work.

lambdalab/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver,sourcegraph/go-langserver,lambdalab/go-langserver,sourcegraph/go-langserver