Add @package_sources for sibling source directories

[talsperre]

Summary

Add @package_sources for including sibling or parent source directories in Metaflow code packages.

This supports project layouts where flows and shared application code live in separate directories:

from metaflow import FlowSpec, package_sources

@package_sources("../src/forecasting")
class TrainFlow(FlowSpec):
    ...

Motivating discussion: https://netflix.slack.com/archives/C02116BBNTU/p1785352517019289

Implementation

• Resolve relative source paths from the flow file directory.
• Support multiple sources, custom archive paths, and additional file suffixes.
• Expose the mutator as metaflow.package_sources.
• Reject unsafe archive paths and missing sources.

Tests

• 649 passed, 1 skipped, 6 deselected
• Pre-commit passes

[greptile-apps]

Greptile Summary

The PR adds a package_sources flow mutator for including files and directories outside a flow’s directory in its code package.

• Resolves relative paths from the flow file’s directory.
• Supports multiple sources, custom archive names, and configurable suffix filters.
• Registers the mutator as a top-level Metaflow API and adds focused unit coverage.

Confidence Score: 4/5

The PR does not appear safe to merge because the previously reported whole-source-root import failure remains unresolved.

The default archive mapping still preserves the src directory prefix, while the new importability test avoids that path by packaging the nested forecasting directory directly; remote code using @package_sources("../src") can therefore still fail to import forecasting.

Files Needing Attention: metaflow/plugins/package_sources.py, test/unit/test_package_sources.py

Important Files Changed

Filename	Overview
metaflow/plugins/package_sources.py	Implements source-spec parsing, archive-path validation, suffix filtering, and package-content generation.
metaflow/plugins/init.py	Registers package_sources as a top-level plugin.
test/unit/test_package_sources.py	Covers top-level exposure, relative source resolution, archive names, suffixes, validation, and missing sources.

_{Reviews (3): Last reviewed commit: "Document package_sources usage" | Re-trigger Greptile}