Add forks to lockfile, don't read them yet

[konstin]

Add the forks to the lockfile, without using them yet, which we'll add in the next PR.

Please review commit-by-commit

Part of #5180 (comment)

[BurntSushi]

This all LGTM. I'll mourn the lock file's simplicity. Markers are everywhere!

[BurntSushi]

Thinking about this a bit more, how does this interact with uv pip compile --universal? We don't write the fork markers to requirements.txt, so does that mean uv pip compile --universal is unstable?

[konstin]

Yes, --universal is unstable, and i don't have a good solution for fixing that:

Example: Packse scenario "preferences-dependent-forking":

First pass of uv pip compile --universal forking.in -o forking.txt:

# This file was autogenerated by uv via the following command:
#    uv pip compile --universal forking.in -o forking.txt
preferences-dependent-forking==0.0.0
    # via -r forking.in
preferences-dependent-forking-bar==1.0.0
    # via
    #   preferences-dependent-forking
    #   preferences-dependent-forking-cleaver
preferences-dependent-forking-cleaver==1.0.0
    # via preferences-dependent-forking
preferences-dependent-forking-foo==1.0.0
    # via
    #   preferences-dependent-forking
    #   preferences-dependent-forking-cleaver
preferences-dependent-forking-foo==2.0.0
    # via preferences-dependent-forking

Second pass of uv pip compile --universal forking.in -o forking.txt:

# This file was autogenerated by uv via the following command:
#    uv pip compile --universal forking.in -o forking.txt
preferences-dependent-forking==0.0.0
    # via -r forking.in
preferences-dependent-forking-bar==1.0.0
    # via
    #   preferences-dependent-forking
    #   preferences-dependent-forking-cleaver
preferences-dependent-forking-cleaver==1.0.0
    # via preferences-dependent-forking
preferences-dependent-forking-foo==1.0.0
    # via
    #   preferences-dependent-forking
    #   preferences-dependent-forking-cleaver