Add support for parsing go mod graph dumps

[arbazkhan971]

Summary

Adds a package datafile handler for the `go mod graph` text format.

• Recognizes `go-mod-graph.deplock` (DepLock-style, same idea as `swift-show-dependencies.deplock` / `pip-inspect.deplock`) and `go.mod.graph`
• Treats the first requiring module as the main module
• Collects unique required modules; those required by the main module are direct, the rest are transitive
• Graph versions are the selected versions, so dependencies are pinned

Test plan

• `pytest tests/packagedcode/test_golang.py` (16 passed)
• `test_recognize_go_mod_graph`
• `test_package_list_command`
• `test_check_datafile_handlers_have_no_duplicate_datasource_id`
• doctests in `go_mod.py`

Fixes #4423

Made with Cursor