[Feature] Support relative parent paths for module files

Description

fga.mod file do not support relative parent paths (e.g. ../model.fga) to module files listed in contents. Files must reside in a child directory of the fga.mod location.

What makes this particularly problematic is that module files must be named fga.mod and cannot have arbitrary names therefore preventing multiple module from being in the same directory.

Benefits

• Enhanced composition flexibility
• Ability to avoid duplication of module definitions

Use Case

As part of a model migration system I aim to support incremental model migrations and need to version models by date while reusing module definitions from previous versions that remain unchanged.

Example

models
├── 20250601
│   ├── core.fga
│   └── fga.mod
├── 20250604
│   ├── wiki.fga
|   └── fga.mod
└── 20250605
    ├── issue-tracker
    │   ├── projects.fga
    |   └── tickets.fga
    └── fga.mod

20250605/fga.mod

schema: '1.2'
contents:
  - ../20250601/core.fga # Not supported
  - ../20250604/wiki.fga # Not supported
  - issue-tracker/projects.fga
  - issue-tracker/tickets.fga

Start by tracing how the CLI reads fga.mod and resolves paths listed under contents. Reproduce the example with parent paths such as ../20250601/core.fga, then inspect the existing handling for child-directory files. Done means valid relative parent paths work without breaking current module loading behavior, with tests covering both cases.