spdx: use go.mod module path for root package name

[jeefy]

Summary

• When generating SBOMs from a directory, PackageFromDirectory now looks for a go.mod file and uses the module path as the root package name instead of the directory basename.
• Fixes the issue where temporary or CI/CD build directories produce meaningless root package names like tmp.b46BF6mTG5 that cascade through all SPDX IDs, file references, and relationships.
• Adds a test covering the nested go.mod lookup path.

Fixes #622

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[k8s-ci-robot]

@jeefy: The following test failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-bom-check-embedded-data	fd4977c	link	true	/test pull-bom-check-embedded-data

Full PR test history. Your PR dashboard. Please help us cut down on flakes by linking to an open issue when you hit one in your PR.

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[puerco]

@jeefy can you rebase this? The failing test was fixed a while ago