feat: support uv virtual

[tdejager]

Description

This add support for uv virtual, which means the package dependencies will be installed but not the package itself. uv switches this by checking if the build-system table exists. I decided to determine the virtual-ness, based on the fact if you have a reference to yourself (something that pixi init adds). The main benefit is that you need less in the pyproject.toml, and that this can be used more as just something to install an environment. Similar as to how the pixi.toml functions instead of always building a project.

The next step is that I want a default initialization for the [tool.pixi.workspace] table, so that you can basically pixi install any pyproject.toml, which would default to a PyPI install. Then when we have pypi -> conda map fully integrated we could then have a flag to switch between a conda or pypi installation.

The current change is pretty simple and basically does two things:

1. Adds support for the uv virtual flag.
2. Enables virtual by default if you do not have a reference to your own project.

How Has This Been Tested?

I need to user-test this a bit, therefore its in draft. Also need to check more off the checklist.

AI Disclosure

• This PR contains AI-generated content.
  • I have tested any AI-generated content in my PR.
  • I take responsibility for any AI-generated content in my PR.

Tools: Opus 4.5

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added sufficient tests to cover my changes.
• I have verified that changes that would impact the JSON schema have been made in schema/model.py.