Fix empty relative dir

[SamuelH91]

Description

The Fast-DDS-Gen is not generating files in relative subdirectories in output directory even when there is no -flat-output-dir flag defined for fastddsgen.

With this change the relative path of the idl file is defined based on the largest common includePath (if there are multiple).

Contributor Checklist

• Commit messages follow the project guidelines.
• Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented.
• ❌ Not checked yet. Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• ❌ Changes are API compatible: The getRelativeDir() API changes as dependant_idl_dir is no more needed. The relative path is defined from the include path(s) and the idl file path. Brakes some test in Fast-DDS-Gen.
• ❌ Not checked yet. Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.