replace bootstrap scheduler with a narrower scope scheduler

[tdyas]

Pants actually sets up two rule graphs: the regular scheduler and a "bootstrap" scheduler which contains the full Python backend (even if the user is not using Python) so that plugin resolution can make use of Pex machinery.

This leads to word-arounds like the BootstrapStatus engine parameter which lets the Python rules figure out which scheduler they are running under and skip raising an exception (e.g., for BUILD file errors) if running in the bootstrap scheduler.

We should remove this complication to start simplifying the plugin resolution process.

[tdyas]

The initial approach in #21986 is unwieldy because of having to create several shell scripts to even allow executing in the Pex-generated venv to work properly.

I am exploring an alternate approach in #21991 by just using uv instead of pex. It resulted in a (thus far) simpler approach as compared to Pex.