apt: relative path for property Filename in Packages (fixes: #586)

[dionysius]

As described in #586

The Nexus Repo generated Packages file does not follow the DebianRepository spec in regard to the Filename: field.

The Filename: field that Nexus Repo generates contains a leading /, this is in direct violation of the spec which requires it to be a relative path (source). The UNIX spec defines a relative path as a path without a leading slash (source).

This creates issues with many apt mirroring utilities.

Uses Paths.relativize() with base / to always output a relative path.

Review advised, unable to test and build due to #600