Follow-up to #6959 and #6961: Use the reachability computation instead
of `propagate_markers` everywhere.
With `marker_reachability`, we have a function that computes for each
node the markers under which it is (`requirements.txt`, no markers
provided on installation) or can be (`uv.lock`, depending on the markers
provided on installation) included in the installation. Put differently:
If the marker computed by `marker_reachability` is not fulfilled for the
current platform, the package is never required on the current platform.
We compute the markers for each package in the graph, this includes the
virtual extra packages and the base packages. Since we know that each
virtual extra package depends on its base package (`foo[bar]` implied
`foo`), we only retain the base package marker in the `requirements.txt`
graph.
In #6959/#6961 we were only using it for pruning packages in `uv.lock`,
now we're also using it for the markers in `requirements.txt`.
I think this closes#4645, CC @bluss.
## Summary
The interface here is intentionally a bit more limited than `uv pip
compile`, because we don't want `requirements.txt` to be a system of
record -- it's just an export format. So, we don't write annotation
comments (i.e., which dependency is requested from which), we don't
allow writing extras, etc. It's just a flat list of requirements, with
their markers and hashes.
Closes#6007.
Closes#6668.
Closes#6670.
Charlie Marsh
konsti