Add --emit-index-url and --emit-find-links to uv export

[Bradley-McCallion]

Closes #10008

Summary

uv export now supports --emit-index-url and --emit-find-links, mirroring the equivalent flags on uv pip compile.

When --emit-index-url is passed, the generated requirements.txt will include --index-url and --extra-index-url directives for any indexes configured in [tool.uv.index]. When --emit-find-links is passed, --find-links entries are included. Neither is emitted by default, preserving existing behavior.

Example output with --emit-index-url:

# This file was autogenerated by uv via the following command:
#    uv export --emit-index-url
--index-url https://pypi.org/simple
--extra-index-url https://download.pytorch.org/whl/cu121

anyio==3.7.0 \
    --hash=sha256:... \
    ...

Test Plan

• Added requirements_txt_emit_index_url and requirements_txt_emit_extra_index_url integration tests
• Also tested with a locally built binary

NOTE This is my first time contributing to uv and I am also new to Rust to all feedback very welcome