Allow relative local directory for repositories of type composer

[helhum]

For a custom mono repo approach, I'm currently generating packages.json file in a local directory.
I would love to be able to specify the path to this repository relative to the root composer.json similar to repositories of type path:

{
    "repositories": [
        {
            "type": "composer",
            "url": "./path/to/local-dir/"
        }
    ]
}

Composer repositories though, only allow URLs. I could specify a file:// URL, but then the specified path needs to be an absolute file path:

{
    "repositories": [
        {
            "type": "composer",
            "url": "file:///absolute/path/to/local-dir/"
        }
    ]
}

Is there anything conceptually ore security wise that is a blocker to allow local relative file system paths for repositories of type Composer?

If not, I'm happy to create a PR to allow it.

[Seldaek]

Ah weird I thought this supported relative paths, but indeed it seems like it doesn't. I think it should be fine to allow it..

[helhum]

Cool, thanks. Then I'll try to provide a PR for that

[helhum]

Added support for local file paths in a very simple way, which I assumed has the lowest impact. Let me know in the PR if this is fine and whether I should add some docs or tests for that.