Support relative includings of requirements files

[Miphreal Adler]

In case of the following structure

├── requirements
│ ├── base.txt
│ ├── local.txt
│ ├── prod.txt
│ └── test.txt
├── requirements.txt

# requirements.txt:
-r requirements/prod.txt

# requirements/prod.txt:
-r base.txt

The current version cannot reach packages in 'base.txt' (though "pip install -r requirements.txt" works properly)

Thanks