Add --script support to uv export for PEP 723 scripts

[charliermarsh]

Summary

You can now run uv export --script main.py to show the dependency tree for a given script. If a lockfile doesn't exist, it will create one.

Closes #8609.

Closes #9657.

[zanieb]

If a lockfile doesn't exist, it will create one.

And it will stick around? I might find that surprising, since it alters future invocations.

[charliermarsh]

Yeah, it will stick around.

[zanieb]

Could we avoid that? Did you have a motivation for that behavior other than simplicity?

[charliermarsh]

I think we can avoid it if you prefer, yeah.

[charliermarsh]

(It seems like a reasonable change to me.)

[zanieb]

I do prefer for now. Thanks!

[charliermarsh]

@zanieb -- I made the same change for uv tree -- we don't persist the lockfile unless it already existed.