perf(debian): use bytes.Index in emptyLineSplit to cut allocation

[paulcacheux]

Description

This PR makes use of bytes.Index in emptyLineSplit instead of strings.Index to remove the extra allocation caused by the bytes to string cast.

Related issues

• Close perf(dpkg): `emptyLineSplit` allocates causing some unnecessary memory allocation in this scanner #7064

Checklist

• I've read the guidelines for contributing to this repository.
• I've followed the conventions in the PR title.
• I've added tests that prove my fix is effective or that my feature works.
• I've updated the documentation with the relevant information (if needed).
• I've added usage information (if the PR introduces new options)
• I've included a "before" and "after" example to the description (if the PR is a user interface change).

[knqyf263]

Thanks!