Feat: support file organization using folder reference

[GMinucci]

What does this PR do

Removes the option for a list of targets in the iOS configuration from variants.yml
Improves PBXGroup creation to allow variants files to be organized in folder reference
Fixes the issues of added files not being included in the main target

How can it be tested

In a new project, run the init command, configure the variants.yml with test values and run the switch command. Once the command is done, confirm that the files are properly added to the .xcodeproj and the build phase is also including the newly generated files.

Task

resolves #244

Additional notes

Reasoning behind removal of list of targets in favor of a single target

Although we have option to provide a list of target this was never implemented. Providing more than one target would lead to unwanted behaviors.
We had the choice to implement the functionality or to remove it completely. If we add it, this means that variants would scale exponentially with the number of targets since variants are applied to all targets. Since the idea of variants is to extend the targets to allow multiple variants, this support would be redundant so we opted to avoid it and remove it completely.
This will also pave the way toward the upcoming functionality related to signing extensions. Since they are also handled as targets in xcode, keeping the target in variants.yml as a list would lead to confusion.

Checklist:

• I ran make validation locally with success
• I have not introduced new bugs
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented on my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new errors

[github-actions]

	Warnings
⚠️	Pull request is relatively big. If this PR contains multiple changes, consider splitting it into separate PRs for easier reviews.

Generated by 🚫 Danger Swift against 77c496e